arg: Marshal GType of 0 to null

Zero is a valid return value from e.g. GObject.type_from_name() in case
the type in question is not registered. We marshal a value of 0
(G_TYPE_INVALID) into JS null.

Closes #11.
8 jobs for 11-handle-0-valued-gtype-gracefully in 19 minutes and 33 seconds (queued for 112 minutes and 33 seconds)
Status Job ID Name Coverage
  Build
passed #4197
fedora_clang

00:10:12

75.2%
passed #4198
fedora_cppcheck

00:00:22

passed #4196
fedora_gcc

00:08:58

failed #4105
fedora_clang

failed #4106
fedora_cppcheck

failed #4104
fedora_gcc

 
  Failure
skipped #4199
on_failure
failed #4107
on_failure