function: Handle callbacks with no return value
function: Handle callbacks with no return value

Previously this was handled by the else clause in this if statement, but
since we switched to rooting, we call .toObjectOrNull() on the callback's
return value. In the case of no return value, the JS return value is
undefined.

We could handle this lower down in the existing else clause, but this
seems easier to understand.

https://bugzilla.gnome.org/show_bug.cgi?id=742249

endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs