Fix compilation of empty resources as generated by MPW's DeRez

[briankendall]

MPW's DeRez command will produce empty resources in this format:

data 'ABCD' (128, "some resource name") {
};

Currently Retro68's reimplementation of Rez will see this as a syntax error. This PR makes a small change to RezParser to allow that syntax.

There are actually situations in classic mac apps where you're supposed to have a completely empty resource, namely when including a FONT resource. Each font family needs to have one extra FONT resource representing the family itself, and it contains no data.

(This is my first time working with Bison grammar, so please double check my work!)

[autc04]

Yes, this will work.

Things might get slightly simpler if you add it as a second alternative to the data clause, such as | "data" res_spec "{" "}" { world.addData($res_spec, ""); } rather than introducing optional_string_expression.

But I'd be happy to merge this as is. Thanks!

[briankendall]

@autc04 That's quite a bit better solution than what I came up with! I went ahead and changed my PR to use that.

[autc04]

Thanks again!