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

Author: javiereguiluz

console/input.rst

@@ -203,3 +203,7 @@ 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``.