Options with [None, "ANY"] do not recognize the None as a default

[markfinal]

As per https://docs.conan.io/en/1.53/reference/conanfile/attributes.html#options

When an option uses "ANY", but its default value is None, then it should be added to the possible option values too.

the dock does not recognise None being the default so

1. it does not render initially correctly
2. setting it to None is treated as a non-default value in the settings, so passed to command lines unnecessarily

An empty string does seem to work in place of None, but is it obvious?