Generating of syntactically incorrect queries when evaluating a service pattern

### Version

5.3.0

### What happened?

I have noticed that during the evaluation of a federated query, Jena performs substitution of a variable for a RDF term even in cases where the given RDF term cannot be valid.

This can be demonstrated by the following federated query:

```sparql
SELECT * WHERE {
VALUES ?P { "literal" }

SERVICE {
?S ?P ?O.
}
}
```

Jena sends the following query to the endpoint:

```sparql
SELECT *
WHERE
{ ?S "literal" ?O }
```

However, such a query is not syntactically correct.

### Relevant output and stacktrace

```shell

```

### Are you interested in making a pull request?

None

Reproduce the issue with the supplied federated SPARQL query, especially the SERVICE pattern and VALUES binding for ?P. Trace the federated-query evaluation that produces the remote query, and consider the result complete when invalid RDF terms are not substituted into syntactically invalid queries sent to the endpoint.

Written by the indexing model from the issue text.