Use proper skipif section

The ODBC tests have to be skipped if no connection to the server can be
established.

Fix #78470: odbc_specialcolumns() no longer accepts $nullable

It is mandatory to pass either `SQL_NO_NULLS` or `SQL_NULLABLE` as
tenth parameter to `SQLSpecialColumns()`; otherwise the f

Fix #78470: odbc_specialcolumns() no longer accepts $nullable

It is mandatory to pass either `SQL_NO_NULLS` or `SQL_NULLABLE` as
tenth parameter to `SQLSpecialColumns()`; otherwise the function call
fails. Therefore the user must be allowed to pass the desired value
as parameter to `odbc_specialcolumns()` again.

Closes GH-6200.

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