fix undefined value not converted to gobject

[CapitaineJSparrow]

Fix #333 (confirmed on my end)

I'm not sure about myself, undefined is not in glib constants so I compare the type of value as string. I'm new to this project and it's the first lines of C code I write since decades so may be not the right fix.

[CapitaineJSparrow]

Added unit test, failing on master but working with my PR