[12.x] collapseWithKeys - Prevent exception in base case (#56002)

* collapseWithKeys - Ensure base case of already flat collection doesn't result in an exception.

* 12.x  Fix style error

* 12.x  Opting to return empty collection in this case instead to ensure functionality of Collection matches LazyCollection

---------

Co-authored-by: Dean Wunder <dean@finao.com.au>

Author: DeanWunder

src/Illuminate/Collections/Collection.php

@@ -165,6 +165,10 @@ public function collapseWithKeys()
             $results[$key] = $values;
         }
 
+        if (! $results) {
+            return new static;
+        }
+
         return new static(array_replace(...$results));
     }
 

tests/Support/SupportCollectionTest.php

@@ -1772,6 +1772,10 @@ public function testCollapseWithKeys($collection)
     {
         $data = new $collection([[1 => 'a'], [3 => 'c'], [2 => 'b'], 'drop']);
         $this->assertEquals([1 => 'a', 3 => 'c', 2 => 'b'], $data->collapseWithKeys()->all());
+
+        // Case with an already flat collection
+        $data = new $collection(['a', 'b', 'c']);
+        $this->assertEquals([], $data->collapseWithKeys()->all());
     }
 
     #[DataProvider('collectionClassProvider')]