PostgreSQL compatibility: `regexp_count` should count empty-pattern matches

Split out from #22247.

### Mismatch

`regexp_count` should count empty-pattern matches.

```sql
SELECT regexp_count('abc', '');
```

PostgreSQL:

```text
4
```

DataFusion:

```text
0
```

### Expected behavior

For PostgreSQL-compatible SQL semantics, DataFusion should either match PostgreSQL's result or raise the same class of domain/semantic error.
