Don't create H groups for empty (null) GUIDs

See https://github.com/hypothesis/lms/pull/6612 for examples of the problem.

[We don't seem to have launches from AI without a GUID.](https://metabase.hypothes.is/question#eyJkYXRhc2V0X3F1ZXJ5Ijp7InR5cGUiOiJuYXRpdmUiLCJuYXRpdmUiOnsicXVlcnkiOiJzZWxlY3QgKiBmcm9tIGFwcGxpY2F0aW9uX2luc3RhbmNlcyB3aGVyZSB0b29sX2NvbnN1bWVyX2luc3RhbmNlX2d1aWQgaXMgbnVsbCBhbmQgbGFzdF9sYXVuY2hlZCA-PSAnMjAyMi0wMS0wMSdcbiAgICAgICAgXG4gICAgICAgIFxuICAgICAgICBcbiIsInRlbXBsYXRlLXRhZ3MiOnt9fSwiZGF0YWJhc2UiOjd9LCJkaXNwbGF5IjoidGFibGUiLCJwYXJhbWV0ZXJzIjpbXSwidmlzdWFsaXphdGlvbl9zZXR0aW5ncyI6e319)

This might be an issue for first launches on application instances where we don't correctly use the GUID from the launch and pick the old (null) value from the DB instead.

We currently check on every launch that the GUID has not changed, also include here a check to avoid cointinue for empty GUIDs just in case.