* Util: Optionally allow empty input in SplitHostPortDefault

Due to a recent change the SQL Server tests failed because passing an
empty datasource url in `util.SplitHostPortDefault` was no more allowed.
This fix contains the following modifications:
- Modifies the util.SplitHostPortDefault not to return an error for empty input.
- Modifies the util.SplitHostPort to return an error for empty input.
- Introduces an additional test for empty input.