Don't use null as exception message

[derrabus]

Q	A
Type	bug
BC Break	no
Fixed issues	N/A

Summary

null is not a valid exception message. Before PHP 8.0, null was silently cast to an empty string. On PHP 8.1 however, passing null as $message to Exception__construct() will trigger a deprecation warning.

This PR proposes to change the tests so that the empty string is used instead of null.

[SenseException]

The values for $message are all the same. Is it necessary to keep them in the data provider?

[derrabus]

There is also one data provider that emits actual error messages.

[morozov]

Note, these changes are likely already implemented in newer branches. But as I said earlier, I do not think we should be implementing the support for PHP 8.1 in DBAL 2.x.

[derrabus]

See my comment #4734 (comment)

Let's continue the discussion there.

[greg0ire]

Thanks @derrabus !