java: prefer null to YYSYMBOL_YYEMPTY

That's one nice benefit from using enums.

* data/skeletons/lalr1.java (YYSYMBOL_YYEMPTY): No longer define it.
Use 'null' instead.
* examples/java/calc/Calc.y, tests/local.at: Adjust.

TODO

@@ -1,4 +1,7 @@
 * Bison 3.6
+** syntax_error_arguments
+Don't store the lookahead if !argv.
+
 ** Documentation
 - yyexpected_tokens in all the languages.
 - YYNOMEM

data/skeletons/lalr1.java

@@ -55,7 +55,7 @@ b4_use_push_for_pull_if([
 m4_define([b4_define_state],[[
     /* Lookahead and lookahead in internal form.  */
     int yychar = YYEMPTY_;
-    SymbolKind yytoken = SymbolKind.YYSYMBOL_YYEMPTY;
+    SymbolKind yytoken = null;
 
     /* State.  */
     int yyn = 0;
@@ -681,7 +681,7 @@ b4_dollar_popdef[]dnl
           {
             ++yynerrs;
             if (yychar == YYEMPTY_)
-              yytoken = SymbolKind.YYSYMBOL_YYEMPTY;
+              yytoken = null;
             yyreportSyntaxError (new Context (yystack, yytoken]b4_locations_if([[, yylloc]])[));
           }
 
@@ -795,7 +795,7 @@ b4_dollar_popdef[]dnl
   {
     /* Lookahead and lookahead in internal form.  */
     this.yychar = YYEMPTY_;
-    this.yytoken = SymbolKind.YYSYMBOL_YYEMPTY;
+    this.yytoken = null;
 
     /* State.  */
     this.yyn = 0;
@@ -972,7 +972,7 @@ b4_dollar_popdef[]dnl
          to an error action in a later state.
     */
     int yycount = 0;
-    if (yyctx.getToken () != SymbolKind.YYSYMBOL_YYEMPTY)
+    if (yyctx.getToken () != null)
       {
         yyarg[yycount++] = yyctx.getToken ();
         yycount += yyctx.getExpectedTokens (yyarg, 1, yyargn);

examples/java/calc/Calc.y

@@ -129,7 +129,7 @@ class CalcLexer implements Calc.Lexer {
     }
     {
       Calc.SymbolKind lookahead = ctx.getToken ();
-      if (lookahead != Calc.SymbolKind.YYSYMBOL_YYEMPTY)
+      if (lookahead != null)
         System.err.print (" before " + ctx.yysymbolName (lookahead));
     }
     System.err.println ("");

tests/local.at

@@ -982,7 +982,7 @@ m4_define([AT_YYERROR_DEFINE(java)],
                       + )["syntax error");
     {
       Calc.SymbolKind token = ctx.getToken ();
-      if (token != Calc.SymbolKind.YYSYMBOL_YYEMPTY)
+      if (token != null)
         System.err.print (" on token @<:@" + ctx.yysymbolName (token) + "@:>@");
     }
     {