fix case

Author: taylorotwell

src/Illuminate/Support/Reflector.php

@@ -21,8 +21,8 @@ public static function isCallable($var, $syntaxOnly = false)
             return is_callable($var, $syntaxOnly);
         }
 
-        if (! isset($var[0]) && ! isset($var[1]) ||
-            ! is_string($var[1])) {
+        if ((! isset($var[0]) || ! isset($var[1])) ||
+            ! is_string($var[1] ?? null)) {
             return false;
         }