Fields.size() should not return -1 [LUCENE-8029]

All call sites of Fields.size() recompute it if it is not available. We should make the API easier to use and require that it never returns -1.

---
Migrated from [LUCENE-8029](https://issues.apache.org/jira/browse/LUCENE-8029) by Adrien Grand (@jpountz)

Start by locating the Fields.size() API, its implementations, and the call sites that currently recompute the value when it is -1. Review how each implementation determines the field count and update the affected callers so the API never returns -1; done means all relevant tests and call sites reflect that contract.

Written by the indexing model from the issue text.