minor symfony#7363 Document that you can't pass empty strings to console options (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
----------

Document that you can't pass empty strings to console options

Related to symfony/symfony#21215

Commits
-------

9666907 Document that you can't pass empty strings to console options

Author: xabbuh

console/input.rst

@@ -206,3 +206,7 @@ You can combine ``VALUE_IS_ARRAY`` with ``VALUE_REQUIRED`` or
     when the option was used without a value (``command --language``) or when
     it wasn't used at all (``command``). In both cases, the value retrieved for
     the option will be ``null``.
+
+    Similarly, due to a PHP limitation, there is no way to pass an empty string
+    as the value of an option. In ``command --prefix`` and ``command --prefix=''``
+    cases, the value of the ``prefix`` option will be ``null``.