[12.x] Simpler and consistent Arr::collapse() (#56842)

* Simpler and consistent `Arr::collapse()`

Removed the negation and `continue` as felt easier to read, consistent with other methods here.

* Update Arr.php

Author: weshooper

src/Illuminate/Collections/Arr.php

@@ -105,12 +105,10 @@ public static function collapse($array)
 
         foreach ($array as $values) {
             if ($values instanceof Collection) {
-                $values = $values->all();
-            } elseif (! is_array($values)) {
-                continue;
+                $results[] = $values->all();
+            } elseif (is_array($values)) {
+                $results[] = $values;
             }
-
-            $results[] = $values;
         }
 
         return array_merge([], ...$results);