Handle empty by-function inputs

[OskarEichler]

Summary

Return the JMESPath-specified empty results before evaluating by-expression types: [] for sort_by and nil for max_by/min_by.

Reproduction

The current implementation evaluates the expression against a missing first element, then raises an invalid-type error. The latest compliance corpus expects empty results without expression evaluation.

Verification

• isolated branch: 1,790 examples, zero failures
• focused empty sort/max/min-by model
• cumulative candidate: 892 current compliance checks, zero failures
• all runtime syntax and Rails 8.1.3.1 integration pass

Compatibility

Nonempty inputs and their homogenous-type validation are unchanged. Prepared with AI-assisted source review; no repository tests were changed.