fix: preserve nullary function metadata

[osipovartem]

Which issue does this PR close?

• Follow-up for complete information_schema function metadata.

Rationale for this change

Functions whose signatures accept zero arguments can currently produce only an untyped placeholder row when TypeSignature::get_example_types() returns no representative input types. This causes nullary scalar/window functions and aggregate signatures such as COUNT() to lose their return type and arity metadata. Nested variadic signatures inside OneOf are also reported as non-variadic.

What changes are included in this PR?

• Resolve a return field for empty argument lists only when the signature explicitly supports zero arguments.
• Apply the same behavior to scalar, aggregate, and window UDF metadata.
• Detect variadic signatures recursively through TypeSignature::OneOf.
• Preserve the variadic marker on OUT-only metadata rows.

The work is limited to information-schema construction and does not affect expression evaluation or query execution hot paths.

Are these changes tested?

SQLLogicTest coverage verifies nullary scalar (current_date), aggregate/nested variadic (count), and window (rank) metadata, including native SHOW FUNCTIONS output.

• cargo test --profile=ci --test sqllogictests -- information_schema
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

Yes. information_schema.routines, information_schema.parameters, and SHOW FUNCTIONS now report return types and variadic arity for zero-argument-compatible functions instead of NULL/incomplete metadata. There are no public API changes.