Revert "Fix the build"

This reverts commit 5d4342dd14bb7ca79b63c1341fdfa420164b35e4.
Revert "Pass 0 (NULL) instead of  for C properties."

This reverts commit db1668a39d9646882d5aa6b7386a3b7ea70f99af.
Revert "gmmproc: Pass 0 (NULL) instead of  to C functions."

This reverts commit 5acce15576a91aec89c04b8fe5f8532b192acb71.
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.