If the QCommandLineOption doesn't have a valueName, the parser won't
read the argument, therefore returning an empty value. If the developers
are calling ::value on the option, they clearly think it's expected to
get a value but won't ever be getting one, so we better warn them about
it.
Change-Id: I434b94c0b817b5d9d137c17f32b92af363f93eb8
Reviewed-by: David Faure <david.faure@kdab.com>