Fix "Odd length text property list" error when property value is nil

The bug occurred because tp--parse-args was incorrectly omitting nil values
from the property list, producing (face) instead of (face nil).

Fixed the same bug pattern in:
- tp--parse-args (used by tp-set, tp-reset, tp-add)
- tp-add-to-layers
- tp-add-to-all-layers

Added 6 new regression tests for nil property values.

Co-authored-by: Kinneyzhang <38454496+Kinneyzhang@users.noreply.github.com>

tp-tests.el

@@ -3766,5 +3766,59 @@ internally, and the final result should have face properties, not tp-palette."
         (should (listp mouse-face-prop))
         (should (memq 'highlight mouse-face-prop))))))
 
+;;; ============================================================
+;;; Nil Value Property Tests (Issue: "Odd length text property list")
+;;; ============================================================
+
+(ert-deftest tp-test-set-with-nil-value ()
+  "Test tp-set with nil value produces valid property list.
+Regression test for: (tp-set \"emacs\" 'face nil) erroring with
+\"Odd length text property list\"."
+  (tp-test-with-temp-buffer
+    (let ((result (tp-set "emacs" 'face nil)))
+      ;; Result should be #("emacs" 0 5 (face nil))
+      (should (stringp result))
+      (should (eq (get-text-property 0 'face result) nil))
+      ;; Verify the property list is valid (has even length)
+      (let ((props (text-properties-at 0 result)))
+        (should (= (% (length props) 2) 0))))))
+
+(ert-deftest tp-test-set-with-nil-value-in-middle ()
+  "Test tp-set with nil value in middle of property list."
+  (tp-test-with-temp-buffer
+    (let ((result (tp-set "emacs" 'face 'bold 'help-echo nil 'display "test")))
+      ;; Result should have face=bold, help-echo=nil, display="test"
+      (should (eq (get-text-property 0 'face result) 'bold))
+      (should (eq (get-text-property 0 'help-echo result) nil))
+      (should (equal (get-text-property 0 'display result) "test")))))
+
+(ert-deftest tp-test-set-with-multiple-nil-values ()
+  "Test tp-set with multiple nil values."
+  (tp-test-with-temp-buffer
+    (let ((result (tp-set "emacs" 'face nil 'help-echo nil)))
+      (should (eq (get-text-property 0 'face result) nil))
+      (should (eq (get-text-property 0 'help-echo result) nil)))))
+
+(ert-deftest tp-test-reset-with-nil-value ()
+  "Test tp-reset with nil value works correctly."
+  (tp-test-with-temp-buffer
+    (let ((result (tp-reset "emacs" 'face nil)))
+      ;; Result should have face=nil
+      (should (eq (get-text-property 0 'face result) nil)))))
+
+(ert-deftest tp-test-add-with-nil-value ()
+  "Test tp-add with nil value works correctly."
+  (tp-test-with-temp-buffer
+    (let ((result (tp-add "emacs" 'face nil)))
+      ;; Result should have face=nil
+      (should (eq (get-text-property 0 'face result) nil)))))
+
+(ert-deftest tp-test-set-nil-value-in-buffer ()
+  "Test tp-set with nil value in buffer region."
+  (tp-test-with-temp-buffer
+    (insert "emacs")
+    (tp-set 1 6 '(face nil))
+    (should (eq (tp-at 1 'face) nil))))
+
 (provide 'tp-ert-tests)
 ;;; tp-ert-tests.el ends here

tp.el

@@ -1055,11 +1055,10 @@ Supports multiple calling conventions:
              (null rest))
         (setq props (list end-or-prop)))
        ;; Standard flat plist: (tp-set "str" 'prop1 val1 'prop2 val2 ...)
+       ;; Always include props-or-val even if it's nil, to handle (tp-set "str" 'prop nil)
        (t
         (setq props (if end-or-prop
-                        (if props-or-val
                         (cons end-or-prop (cons props-or-val rest))
-                          (list end-or-prop))
                       nil)))))
      ;; First arg is a number - region convention
      ((numberp start-or-string)
@@ -3924,11 +3923,9 @@ Returns the modified object (string) or nil for buffer operations."
             start 0
             end (length start-or-string))
       ;; Construct plist from end-or-plist, plist-or-object, and rest
+      ;; Always include plist-or-object even if nil, to handle (... 'prop nil)
       (when end-or-plist
-        (setq plist (cons end-or-plist
+        (setq plist (cons end-or-plist (cons plist-or-object rest)))))
-                          (if plist-or-object
-                              (cons plist-or-object rest)
-                            rest)))))
      ;; Region form: (tp-add-to-layers ids start end plist object)
      ((numberp start-or-string)
       (setq start start-or-string
@@ -3996,11 +3993,9 @@ Returns the modified object (string) or nil for buffer operations."
             start 0
             end (length start-or-string))
       ;; Construct plist from end-or-plist, plist-or-object, and rest
+      ;; Always include plist-or-object even if nil, to handle (... 'prop nil)
       (when end-or-plist
-        (setq plist (cons end-or-plist
+        (setq plist (cons end-or-plist (cons plist-or-object rest)))))
-                          (if plist-or-object
-                              (cons plist-or-object rest)
-                            rest)))))
      ;; Region form: (tp-add-to-all-layers start end plist object)
      ((numberp start-or-string)
       (setq start start-or-string