viml/parser/expressions: Finish parser

Note: formatc.lua was unable to swallow some newer additions to ExprASTNodeType 
(specifically `kExprNodeOr = '|'` and probably something else), so all `= …` 
were dropped: in any case they only were there in order to not bother updating 
viml_pexpr_debug_print_ast_node and since it is now known all nodes which will 
be present it is not much of an issue.

src/nvim/viml/parser/parser.h

@@ -173,7 +173,7 @@ static inline void viml_parser_highlight(ParserState *const pstate,
                                          const size_t len,
                                          const char *const group)
 {
-  if (pstate->colors == NULL) {
+  if (pstate->colors == NULL || len == 0) {
     return;
   }
   // TODO(ZyX-I): May do some assert() sanitizing here.