Address code review: add seq require and improve nil checks

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

tp.el

@@ -31,6 +31,7 @@
 
 (require 'cl-lib)
 (require 'dash)
+(require 'seq)
 
 ;;; tp layer define
 
@@ -69,16 +70,18 @@ For multiple layers, they are stored as a group in `tp-layer-groups'.
 The first layer in the definition is the top layer."
   (declare (indent defun))
   ;; Determine if this is a single layer (one plist argument) or multiple layers
+  (let ((first-layer (car layers)))
     (let ((is-single-layer
            (and (= (length layers) 1)
-              (listp (car layers))
+                first-layer
+                (listp first-layer)
                 ;; A plist has an even number of elements (key-value pairs)
-              (cl-evenp (length (car layers)))
+                (cl-evenp (length first-layer))
                 ;; The first element is a property name (symbol, not nil)
-              (symbolp (caar layers)))))
+                (symbolp (car first-layer)))))
       (if is-single-layer
           ;; Single layer: (tp-define-layer name '(plist...))
-        (let ((properties (car layers)))
+          (let ((properties first-layer))
             `(progn
                (if (assoc ',name tp-layer-alist)
                    (setf (cdr (assoc ',name tp-layer-alist)) ',properties)
@@ -109,7 +112,7 @@ The first layer in the definition is the top layer."
            (if (assoc ',name tp-layer-groups)
                (setf (cdr (assoc ',name tp-layer-groups)) ',layer-names)
              (push (cons ',name ',layer-names) tp-layer-groups))
-           (assoc ',name tp-layer-groups))))))
+           (assoc ',name tp-layer-groups)))))))
 
 (defun tp-layer-props (layer-name)
   "Return properties for layer LAYER-NAME from `tp-layer-alist'.