support strings for empty?

README.md

@@ -271,6 +271,7 @@ sqrt  | `(sqrt 2)` | `1.41` | Calculate the square root.
 
 Name    | Example | Example result | Description
 ------- | ------- | -------------- | -----------
+empty?  | `(empty? "")` | `true` | Return true if argument is empty string.
 len     | `(len "hello world")` | `11` | Return the length of a string using [strlen](https://www.php.net/manual/en/function.strlen.php).
 trim    | `(trim " abc ")` | `"abc"` | Trim the string using [trim](https://www.php.net/manual/en/function.trim).
 upcase  | `(upcase "abc")` | `"ABC"` | Make the string upper case using [strtoupper](https://www.php.net/manual/en/function.strtoupper).

src/Lib/Collections.php

@@ -45,7 +45,15 @@ class Collections implements ILib
         // Read information
 
         $env->set('empty?', new CoreFunc('empty?', 'Return true if collection is empty.', 1, 1,
-            fn (Collection $a) => $a->count() == 0
+            function ($a) {
+                if ($a instanceof Collection) {
+                    return $a->count() === 0;
+                } elseif (is_string($a)) {
+                    return $a === '';
+                }
+
+                throw new MadLispException('argument to empty? is not collection or string');
+            }
         ));
 
         $env->set('get', new CoreFunc('get', 'Get the item from first argument (collection) by using the second argument as index or key.', 2, 2,