fix: function signatures should have correct nullabilitiy

[Broshen]

Some function signatures in functions_comparison.yaml describe that their response can be null, but the type in return doesn't have a ?, indicating that it is non-nullable. This PR fixes the function signature to match the description that responses can be NULL.

[github-actions]

ACTION NEEDED

Substrait follows the Conventional Commits
specification for
release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

[vbarua]

So the ? at the end here doesn't mean anything unless you set the Nullability Handling to DECLARED_OUTPUT or DISCRETE. MIRROR, the default, ignores it because the output nullability is determined by the input type nullabilities.

[tokoko]

@Broshen the default mode (MIRROR) implies that output is nullable if any of the inputs is nullable which is the intention here I think (?).

Setting it to DECLARED_OUTPUR + ? is not correct because function output will have to be always treated as nullable, even if inputs are known not to contain nulls.