[JAVA] Vector should avoid reallocating validity buffer when necessary

Vector always reallocates validity buffer in the case of all-null or non-null

Ideally we should only allocate if the source was not provided

**Reporter**: [Zhenyuan Zhao](https://issues.apache.org/jira/browse/ARROW-3222) / @zzhao0

_{**Note**: *This issue was originally created as [ARROW-3222](https://issues.apache.org/jira/browse/ARROW-3222). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the allocation paths at BaseFixedWidthVector.java#L511 and BitVectorHelper.java#L180, then trace how validity buffers are handled for all-null and non-null vectors. The change is complete when allocation occurs only when the source was not provided, while avoiding unnecessary validity-buffer reallocation.

Written by the indexing model from the issue text.