gmmproc: Pass NULL to C function calls for optional empty strings.
gmmproc: Pass NULL to C function calls for optional empty strings.

	* tools/pm/Output.pm (convert_args_cpp_to_c): When generating
	conversions for the C++ arguments, if the argument is optional
	and it is a std::string or a Glib::ustring, prepend a check for
	the empty string to the standard conversion, passing NULL to the
	C function if the string is empty.

	Bug #686909.

GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm