Properly handles 7.4 warning in case where all incompatible modules disabled

This allows users of earlier versions of Vim (who cannot upgrade for whatever reason) to stop the somewhat annoying "these modules require vim >= 7.4 and have been disabled" warning on startup, by disabling all said modules manually like this:

	let g:pandoc#modules#disabled = ["bibliographies", "command"]

Without this changeset, attempting to do the above will produce an error because `s:module_disabled` is undefined when the conditional testing its value is reached.