dbapi: try to restore compatibility of the connect api with 3.15
A few issues here:
- urlparse('foo').scheme is '', not None
- cnxprops is an optional argument
Regression from 62213a34726e ("[db-api/configuration] simplify db-api
and configuration so that all the connection information is in the
repository url, closes #2521848")
Closes #2754322
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