@Override
        public Object apply(final String s, final GremlinParser.VariableContext variableContext) {
            return null;
        }
This {@link VariableResolver} simply provides a {@code null} value for all variable names. It's typical use
is for when you really don't intend to execute the traversal and just want to get an instance of one when
bindings are being used as with {@link NoOpTerminalVisitor}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/grammar/VariableResolver.java