Drop rows with missing xref_id from xref details

[bfoltyn]

@dhimmel I've noticed that sometimes xref_id is missing in xref details. For example:

`EFO:0007308`

```json "xref_details": [ { "xref_id": "DOID:0080158", "sources": [ "MONDO:equivalentTo" ], "relation": "skos:exactMatch" }, { "xref_id": null, "sources": null, "relation": null }, .... ```

I've added a fix to remove rows where xref_id is missing from xref details.

[bfoltyn]

@dhimmel When triggering GH action, should i check Overwrite output on an existing branch. ?

[dhimmel]

When triggering GH action, should i check Overwrite output on an existing branch

Yes, check that otherwise the outputs will not be committed to the output branch.