add phpstan assertions to Collection isEmpty and isNotEmpty (#51998)

* add @phpstan-assert-if-true|false for Collection isEmpty/isNotEmpty

* fix code style

Author: johanrosenson

src/Illuminate/Collections/Collection.php

@@ -663,6 +663,10 @@ public function intersectByKeys($items)
     /**
      * Determine if the collection is empty or not.
      *
+     * @phpstan-assert-if-true null $this->first()
+     *
+     * @phpstan-assert-if-false !null $this->first()
+     *
      * @return bool
      */
     public function isEmpty()

src/Illuminate/Collections/Traits/EnumeratesValues.php

@@ -367,6 +367,10 @@ public function ensure($type)
     /**
      * Determine if the collection is not empty.
      *
+     * @phpstan-assert-if-true !null $this->first()
+     *
+     * @phpstan-assert-if-false null $this->first()
+     *
      * @return bool
      */
     public function isNotEmpty()