fix(concat): correct nullability inference (nullable only if all arguments nullable)

[ujjwaltwri]

Fixes #19171

Problem

The concat scalar function currently reports its result as always nullable at
planning/schema time. However, its runtime semantics are:

• concat ignores NULL inputs.
• The result becomes NULL only if all input arguments are NULL.

This mismatch causes incorrect nullability in inferred schemas and can affect
optimizer behavior.

What this PR does

Implements return_field_from_args for ConcatFunc so that:

1. The return DataType is derived using the existing return_type logic.
2. The return field’s nullability is computed as:

(If there are no argument fields, the return is considered nullable defensively.)

This aligns schema-time nullability with runtime behavior and matches the
semantics used by other SQL engines.

Tests

• All existing unit tests for concat pass.
• Parquet & CSV tests verified locally after initializing test submodules.
• No behavior changes to runtime concatenation: only planner-side metadata improved.

Notes

If CI finds any compatibility adjustments required across DataFusion crates,
I will update this PR accordingly.

[ujjwaltwri]

Happy to adjust anything if reviewers prefer a different nullability rule or want
the implementation placed elsewhere. The change is planner-only and does not
modify runtime behavior.

[Jefffrey]

This seems to be modifying the DF version instead of the Spark version?

https://github.com/apache/datafusion/blob/83736efc4ad8865019b0809ac9d87e63eabbe0a8/datafusion/spark/src/function/string/concat.rs