[AURON #1579] Fix and enhance nvl2 unit test

[yew1eb]

Which issue does this PR close?

Closes #1579 .

Rationale for this change

nvl2(expr1, expr2, expr3) semantics:

• if expr1 is not null → return expr2
• if expr1 is null → return expr3

The original nvl2 test was incorrect and asserted the wrong expected result:

      sql(
        "create table t1 using parquet as select 'base'" +
          " as base, 3 as exponent")
      val functions =
        """
          |select
          |  nvl2(null, base, exponent), nvl2(4, base, exponent)
          |from t1
                      """.stripMargin

      val df = sql(functions)
      checkAnswer(df, Seq(Row("base", 3)))   // ← WRONG!

Original test incorrectly expected Row("base", 3) but actual result is Row(3, "base").

What changes are included in this PR?

• Removed the original incorrect nvl2 test.
• Replaced with correct and strengthened test case.

Are there any user-facing changes?

No.

How was this patch tested?

Unit tests passed.

[merrily01]

LGTM

[ShreyeshArangath]

Thanks, @yew1eb. This will help us close #1595