[10.x] fix handle shift() on an empty collection (#51841)

* add test for collection shift on a empty collection

* fix collection shift when dealing with an empty collection

* place the `isEmpty()` check before the count check

* update naming and assert the actual values

Author: Treggats

src/Illuminate/Collections/Collection.php

@@ -1134,7 +1134,11 @@ public function shift($count = 1)
             throw new InvalidArgumentException('Number of shifted items may not be less than zero.');
         }
 
-        if ($count === 0 || $this->isEmpty()) {
+        if ($this->isEmpty()) {
+            return null;
+        }
+
+        if ($count === 0) {
             return new static;
         }
 

tests/Support/SupportCollectionTest.php

@@ -404,6 +404,23 @@ public function testShiftReturnsAndRemovesFirstXItemsInCollection()
         (new Collection(['foo', 'bar', 'baz']))->shift(-2);
     }
 
+    public function testShiftReturnsNullOnEmptyCollection()
+    {
+        $itemFoo = new \stdClass();
+        $itemFoo->text = 'f';
+        $itemBar = new \stdClass();
+        $itemBar->text = 'x';
+
+        $items = collect([$itemFoo, $itemBar]);
+
+        $foo = $items->shift();
+        $bar = $items->shift();
+
+        $this->assertSame('f', $foo?->text);
+        $this->assertSame('x', $bar?->text);
+        $this->assertNull($items->shift());
+    }
+
     /**
      * @dataProvider collectionClassProvider
      */