DX-85876: Failure in UnionReader.read after DecimalVector promotion to UnionVector

[sgcowell]

When a DecimalVector is promoted to a UnionVector via a
PromotableWriter, the UnionVector will have the decimal vector in it's
internal struct vector, but the decimalVector field will not be set.
If UnionReader.read is then used to read from the UnionVector, it will
fail when it tries to read one of the promoted decimal values, due
to decimalVector being null, and the exact decimal type not being
provided. This failure is unnecessary though as we have a pre-existing
decimal vector, the caller just does not know the exact type - and
it shouldn't be required to.

The change here is to check for a pre-existing decimal vector in the
internal struct when getDecimalVector() is called. If one exists,
set the decimalVector field and return. Otherwise, if none exists,
throw the exception.

[github-actions]

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