fix: Cpp $rule.text fail if $rule is nullptr

With a grammar like this:

```antlr
rule: (VAL | VAR {
        is_constant = false;
    }) name=IDENTIFIER (COLON type {
        type_name = $type.text; <-- we look this line
    })? (EQ value=expression {
        value = $value.tree;
    })?
```

When generating for Cpp target we get this specific result:

```cpp
type_name = (antlrcpp::downCast<Variable_declarationContext *>(_localctx)->typeContext != nullptr ? _input->getText(antlrcpp::downCast<Variable_declarationContext *>(_localctx)->typeContext->start, antlrcpp::downCast<Variable_declarationContext *>(_localctx)->typeContext->stop) : nullptr);
```

If input doesn't match the `type` branch then we fallback on nullptr. As
of cpp23 standard this line doesn't compile as it waits for
`std::string`.

AS `Token::getText` returns `std::string`, then fallback value should be
empty string

Signed-off-by: Kevin Traini <kevin@ktraini.com>

Author: Gashmob

tool/resources/org/antlr/v4/tool/templates/codegen/Cpp/Cpp.stg

@@ -979,7 +979,7 @@ RulePropertyRef_stopHeader(r)  ::= "<! Required but unused. !>"
 RulePropertyRef_stop(r)  ::= "(<ctx(r)>-><r.label> != nullptr ? (<ctx(r)>-><r.label>->stop) : nullptr)"
 
 RulePropertyRef_textHeader(r)  ::= "<! Required but unused. !>"
-RulePropertyRef_text(r)  ::= "(<ctx(r)>-><r.label> != nullptr ? _input->getText(<ctx(r)>-><r.label>->start, <ctx(r)>-><r.label>->stop) : nullptr)"
+RulePropertyRef_text(r)  ::= "(<ctx(r)>-><r.label> != nullptr ? _input->getText(<ctx(r)>-><r.label>->start, <ctx(r)>-><r.label>->stop) : \"\")"
 
 RulePropertyRef_ctxHeader(r)   ::= "<! Required but unused. !>"
 RulePropertyRef_ctx(r)   ::= "<ctx(r)>-><r.label>"