Since this behavior is changed back and forth from version to version, it's probably better to make it optional, and leave current behavior default in current(3.1) series, and new/old one to be default in the next(3.2) series.

Current behavior is justified by this:
https://docs.microsoft.com/en-us/sql/odbc/reference/develop-app/arguments-in-catalog-functions says, that SQLTables arguments are pattern values, and if you follow the link for pattern values description https://docs.microsoft.com/en-us/sql/odbc/reference/develop-app/pattern-value-arguments, you can read there "Passing a null pointer to a search pattern argument does not constrain the search for that argument; that is, a null pointer and the search pattern % (any characters) are equivalent.". This quote suggests, that our (current) interpretation is correct, and have the right to stay at least as optional. Especially since it was in all 3.1 ga releases.

The option NULLISCURRENT has been added. If it's set, new(that also happens to be old) behavior is enforced, and NULL catalog value is treated as "currently selected schema". Otherwise NULL means "any schema" as it is now. I'll probably give another think what should be the default for this option in the 3.2. Changing behavior back and forth is not the best idea as well.

"NULL catalog parameter means current schema" has been made default behavior in 3.2.0