gom: Fix a format warning

By casting GTypes to ints to print them.
gom: Better warnings for _set_primary_key()

By printing the class name that we're modifying.
gom: Allow the empty string as an ID

gjs doesn't allow null values for string properties, so work-around that
by allowing '0' as a value.

https://bugzilla.gnome.org/show_bug.cgi?id=728301
gom: Don't assert when the primary key isn't set

gjs will want to set the primary key after the object has been
constructed. We'll assert() at the first use of the resource instead.

https://bugzilla.gnome.org/show_bug.cgi?id=728301