Describe the bug, including details regarding any error messages, version, and platform.

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.