[SPARK-56840][SQL] Avoid unresolved NullIf type lookup

[sunchao]

Why are the changes needed?

NULLIF builds its replacement expression before analysis has resolved all child expressions.
For nested field references, the existing implementation can read the left operand's data type
too early while constructing the null branch, which can fail analysis even though the SQL shape
is valid.

SPARK-56840 tracks this analyzer failure.

What changes were proposed in this PR?

• Build the NULLIF null branch with a lazy typed-null placeholder so construction does not eagerly
  read the unresolved left operand type, while NullIf.replacement.dataType remains valid once the
  operand type is available.
• Make that placeholder RuntimeReplaceable, so ReplaceExpressions restores an ordinary typed
  Literal(null, ...) before later optimizer rules run and existing null-literal simplifications
  continue to apply.
• Add focused regressions for:
  • nested struct-field nullif(c.provider, lower(...)) analysis in both
    ALWAYS_INLINE_COMMON_EXPR modes;
  • NullIf replacement type reporting before type coercion;
  • optimizer replacement back to a normal null literal;
  • explain output avoiding exposure of the internal helper name.

Does this PR introduce any user-facing change?

Yes. Valid NULLIF expressions over unresolved nested field references that could fail during
analysis now resolve and execute successfully.

How was this patch tested?

• build/sbt 'catalyst/testOnly org.apache.spark.sql.catalyst.expressions.NullExpressionsSuite -- -z "NullIf replacement preserves its data type before type coercion"'
• build/sbt 'catalyst/testOnly org.apache.spark.sql.catalyst.optimizer.OptimizerSuite -- -z "NullIf typed null branch is replaced with a null literal"'
• build/sbt 'sql/testOnly org.apache.spark.sql.DataFrameFunctionsSuite -- -z "nullif function"'
• build/sbt 'sql/testOnly org.apache.spark.sql.ExplainSuite -- -z "explain for these functions; use range to avoid constant folding"'

Was this patch authored or co-authored using generative AI tooling?

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