-Wempty-rule: diagnose empty rules without %empty

* src/complain.h, src/complain.c (warning_empty_rule, Wempty_rule):
New warning category.
(warnings_args, warnings_types): Adjust.
* src/reader.c (grammar_rule_check): Check the empty rules are
flagged by %empty.
* tests/actions.at (Implicitly empty rule): New.
* tests/existing.at: Add expected warnings.

src/complain.c

@@ -63,6 +63,7 @@ static const char * const warnings_args[] =
   "conflicts-sr",
   "conflicts-rr",
   "deprecated",
+  "empty-rule",
   "precedence",
   "other",
   "all",
@@ -78,6 +79,7 @@ static const int warnings_types[] =
   Wconflicts_sr,
   Wconflicts_rr,
   Wdeprecated,
+  Wempty_rule,
   Wprecedence,
   Wother,
   Wall,