GateWatcher handles stack trace caller not found (#1128)

For an app using Swoole/Laravel Octane and Telescope configured for
`config('telescope.ignore_packages') === true`, method
getCallerFromStackTrace() will return null.

UI file telescope/resources/js/screens/gates/preview.vue will hide
caller file location details if they're not stored in the Telescope
entry so save that path as null.

Also fix the getCallerFromStackTrace() @return docblock as that method
was written before Laravel Octane magic was conjured up.

src/Watchers/FetchesStackTrace.php

@@ -10,7 +10,7 @@ trait FetchesStackTrace
      * Find the first frame in the stack trace outside of Telescope/Laravel.
      *
      * @param  string|array  $forgetLines
-     * @return array
+     * @return array|null
      */
     protected function getCallerFromStackTrace($forgetLines = 0)
     {

src/Watchers/GateWatcher.php

@@ -58,8 +58,8 @@ public function recordGateCheck(?Authenticatable $user, $ability, $result, $argu
             'ability' => $ability,
             'result' => $this->gateResult($result),
             'arguments' => $this->formatArguments($arguments),
-            'file' => $caller['file'],
-            'line' => $caller['line'],
+            'file' => $caller['file'] ?? null,
+            'line' => $caller['line'] ?? null,
         ]));
 
         return $result;