Fix return type docblock for the Cells::get()

PHPStorm inspections and PHPStan display an incorrect hint or
error if the special type `null` is not specified.

Closes #1398

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ### Fixed
 
 - Fix Chart samples by updating chart parameter from 0 to DataSeries::EMPTY_AS_GAP [#1448](https://github.com/PHPOffice/PhpSpreadsheet/pull/1448)
+- Fix return type in docblock for the Cells::get() [#1398](https://github.com/PHPOffice/PhpSpreadsheet/pull/1398)
 
 ### Changed
 

src/PhpSpreadsheet/Collection/Cells.php

@@ -426,7 +426,7 @@ class Cells
      *
      * @param string $pCoord Coordinate of the cell
      *
-     * @return \PhpOffice\PhpSpreadsheet\Cell\Cell Cell that was found, or null if not found
+     * @return null|\PhpOffice\PhpSpreadsheet\Cell\Cell Cell that was found, or null if not found
      */
     public function get($pCoord)
     {