Fix tryBase64Decode() with invalid input

[rschu1ze]

See #54119 (comment)

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Function tryBase64Decode() now returns an empty string instead of garbage if the input is invalid, e.g. SELECT tryBase64Decode('aoeo054640eu='); (note the padding character = at the end which corrupts the base64 value). This makes the function behavior consistent with the behavior that has been documented for many years. At the same time, this marks an incompatibility with the previously incorrect function behavior.

[robot-ch-test-poll]

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[rschu1ze]

Backward compatibility test failed as expected, I updated the changelog entry.

[zvonand]

test_backward_compatibility/test_functions.py::test_string_functions is failing all the way after this PR, also in master. Is this OK?

[rschu1ze]

test_backward_compatibility/test_functions.py::test_string_functions is failing all the way after this PR, also in master. Is this OK?

Nope, fix: #56975

[tavplubix]

@rschu1ze, please check failed tests before merging. Even if failures seem to be unrelated to changes, they must be debugged anyway