allUsers may error if owner is null

`Team->allUsers()` may error out, if `$this->owner` is `null`. This may happen if user was deleted (team owner has no foreign index). In this case `$this->users->merge([null])` which errors.

src/Team.php

@@ -23,7 +23,9 @@ public function owner()
      */
     public function allUsers()
     {
-        return $this->users->merge([$this->owner]);
+        return $this->owner ? 
+            $this->users->merge([$this->owner]) :
+            $this->users;
     }
 
     /**