bug #40610 Fixed bugs found by psalm (Nyholm)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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Fixed bugs found by psalm

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

When running psalm for branch 4.4, we get around 1500 errors. I browsed though them all and here is a small PR with the actual bugs I think should be fixed.

I'll make review comments about them below.

Commits
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0a0cd7a274 Fixed bugs found by psalm

Author: nicolas-grekas

Definition.php

@@ -102,7 +102,7 @@ public function setChanges(array $changes)
     /**
      * Sets a factory.
      *
-     * @param string|array|Reference $factory A PHP function, reference or an array containing a class/Reference and a method to call
+     * @param string|array|Reference|null $factory A PHP function, reference or an array containing a class/Reference and a method to call
      *
      * @return $this
      */