HIVE-28503: Vectorization: concat (||) on string columns returns NULL for non-null inputs

[maheshrajus]

What changes were proposed in this pull request?

• In StringGroupConcatColCol.evaluate(), on the "no nulls in either input" branch, set outV.noNulls = true before writing. (Matches what the sibling StringGroupColConcatStringScalar / StringScalarConcatStringGroupCol already do)
• Added two JUnit tests in TestVectorStringConcat that pre-set outV.noNulls = false (simulating a prior batch that carried NULLs), then assert the flag is restored after evaluate()

Why are the changes needed?

The output BytesColumnVector(outV) is reused across batches. A previous batch that went through a null-carrying branch leaves outV.noNulls = false. The no-nulls branch wrote per-row isNull[i] = false but never reset outV.noNulls = true, so downstream vectorized consumers that utilizes on noNulls treated fully-populated rows as NULL and producing NULL for a || b on rows where both operands were non-null.

More code level details we can find in the jira: https://issues.apache.org/jira/browse/HIVE-28503

Does this PR introduce any user-facing change?

No

How was this patch tested?

With added unit tests

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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