[gi] Do not require signature for D-BUS methods without arguments

Calling methods on DBusProxy objects usually requires specifying the signature
as first argument. However, if the D-BUS method does not take any arguments,
specifying the empty '()' signature does not give any additional information,
so allow the caller to just call the proxy method without any arguments.

Also ensure that passing a non-string signature raises a comprehensible
exception, instead of crashing deep in the GVariant leaf constructor.

https://bugzilla.gnome.org/show_bug.cgi?id=644260