Fix the indexing of symbolic names of lexer rules in parser

If a combined grammar doesn't contain any lexer rule definitions
but inline literal constants only, then the symbolicName list of
the parser is empty. The patch prepares the parser of Grammarinator
for such cases.

grammarinator/tool/parser.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2018-2023 Renata Hodovan, Akos Kiss.
+# Copyright (c) 2018-2024 Renata Hodovan, Akos Kiss.
 #
 # Licensed under the BSD 3-Clause License
 # <LICENSE.rst or https://opensource.org/licenses/BSD-3-Clause>.
@@ -168,7 +168,7 @@ def _antlr_to_grammarinator_tree(self, antlr_node, parser, visited=None):
                 depth = max(depth, child_depth + 1)
         else:
             assert isinstance(antlr_node, TerminalNode), f'An ANTLR node must either be a ParserRuleContext or a TerminalNode but {antlr_node.__class__.__name__} was found.'
-            name, text = (parser.symbolicNames[antlr_node.symbol.type], antlr_node.symbol.text)
+            name, text = parser.symbolicNames[antlr_node.symbol.type] if len(parser.symbolicNames) >= antlr_node.symbol.type else '<INVALID>', antlr_node.symbol.text
             assert name, f'{name} is None or empty'
 
             if not self._hidden: