In Bison 1.875's yacc.c, YYLLOC_DEFAULT was called regardless

In Bison 1.875's yacc.c, YYLLOC_DEFAULT was called regardless
whether the reducion was empty or not.  This leaves room to
improve the use of YYLLOC_DEFAULT in such a case.
lalr1.cc is still experimental, so changing this is acceptable.
And finally, there are probably not many users who changed the
handling of locations in GLR, so changing is admissible too.
* data/glr.c, data/lalr1.cc, data/yacc.c (YYLLOC_DEFAULT): On an
empty reduction, set @$ to an empty location ending the previously
stacked symbol.
Adjust uses to make sure the code is triggered on empty
reductions.
* tests/actions.at (_AT_CHECK_PRINTER_AND_DESTRUCTOR): Adjust the
expected output: empty reductions have empty locations.