yacc: fix handling of CPP guards when no header is generated

yacc: fix handling of CPP guards when no header is generated

When no header was to be generated, Bison would issue:

  /* In a future release of Bison, this section will be replaced
     by #include "".  */
  #ifndef YY_
  # define YY_

It now properly generates nothing.

* data/c.m4 (b4_cpp_guard_open, b4_cpp_guard_close): Issue nothing when
the file name is empty.
* data/yacc.c: Do not generate the above comment when there is no header
to generate.
* NEWS: Update.