Lexmatch: inconsistent behaviour in `sqlite` and `default` mode

According to documentation, I should be able to use `lexmatch` with other ontology interfaces. However, while this here works fine:

```
runoak -i sqlite:foodon_wd.owl lexmatch -o foodon_wd_lexmatch.tsv
```

This here:

```
runoak -i foodon_wd.owl lexmatch -o foodon_wd_lexmatch.tsv
```

results in an empty mapping set data frame (with curie map and all, just no mappings).

The following warnings are printed:

```
WARNING:root:Using rdflib rdf/xml parser; this behavior may change in future
WARNING:root:Not a curie: foodon.owl
WARNING:root:Not a curie: foodon.owl
```

One idiosyncrasy of the ontology in question is that contains entities with no OWL typing in it:

```

cashew apple juice

```

Start by running the two documented runoak lexmatch commands and compare their mapping sets and warnings. Trace how the sqlite and default ontology interfaces handle the RDF/XML input, including the untyped rdf:Description entity. Done means default mode produces the expected mappings consistently with sqlite mode.

Written by the indexing model from the issue text.