Allow `%empty` to be used to highlight empty productions.

[ltratt]

This is a Bison-ism, but it seems reasonable to allow it even in "Yacc" mode.

Addresses one suggestion from https://github.com/softdevteam/grmtools/issues/129.

[ratmice]

This looks good to me, and now I can get rid of a bunch of Empty:; rules :+1: bors r+

[bors[bot]]

Build succeeded:

• buildbot/buildbot-build-script