minor #15194 Update framework.router.strict_requirements (AntoineRoue)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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Update framework.router.strict_requirements

This value was not changed with symfony/symfony#30390

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Commits
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90e6580 Update framework.rst

Author: wouterj

reference/configuration/framework.rst

@@ -1221,7 +1221,7 @@ The value can be one of:
 ``true``
     Throw an exception when the requirements are not met;
 ``false``
-    Disable exceptions when the requirements are not met and return ``null``
+    Disable exceptions when the requirements are not met and return ``''``
     instead;
 ``null``
     Disable checking the requirements (thus, match the route even when the