minor symfony#25656 Update return type in docblock. (ADmad)

This PR was submitted for the 3.4 branch but it was merged into the 2.7 branch instead (closes symfony#25656).

Discussion
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Update return type in docblock.

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Commits
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c5f0f4c Update return type in docblock.

Author: nicolas-grekas

src/Symfony/Component/Console/Input/InputInterface.php

@@ -21,7 +21,7 @@ interface InputInterface
     /**
      * Returns the first argument from the raw parameters (not parsed).
      *
-     * @return string The value of the first argument or null otherwise
+     * @return string|null The value of the first argument or null otherwise
      */
     public function getFirstArgument();