php 7.4 fixes

Author: ArtemGoutsoul

src/CoffeeScript/Lexer.php

@@ -1229,7 +1229,7 @@ function tag($index = 0, $tag = NULL)
       $token[0] = $tag;
     }
 
-    return $token[0];
+    return $token[0] ?? null;
   }
 
   function tag_parameters()