Fix php8.2 deprecation: mb_strwidth():

- Passing null to parameter laravel#1 ($string) of type string is deprecated
- add test

Author: tivvie

src/Illuminate/Support/Str.php

@@ -721,6 +721,10 @@ public static function length($value, $encoding = null)
      */
     public static function limit($value, $limit = 100, $end = '...', $preserveWords = false)
     {
+        if ($value === null) {
+            return $value;
+        }
+
         if (mb_strwidth($value, 'UTF-8') <= $limit) {
             return $value;
         }

tests/Support/SupportStrTest.php

@@ -766,6 +766,8 @@ public function testLimit()
         $this->assertSame('这是一...', Str::limit($nonAsciiString, 6, preserveWords: true));
         $this->assertSame('这是一', Str::limit($nonAsciiString, 6, ''));
         $this->assertSame('这是一', Str::limit($nonAsciiString, 6, '', true));
+
+        $this->assertSame(null, Str::limit(null));
     }
 
     public function testLength()