ARROW-13645 [Java]: Allow NullVectors to have distinct field names

[liyafan82]

As discussed in the ML (https://lists.apache.org/thread.html/r19aad8a34f63334d3fcd627106f69be13f689740b83930a4eecc17b3%40%3Cdev.arrow.apache.org%3E), the current implementation use hard-coded field names. This may cause some problems, for example, when reconstruct a usable schema from a list of FieldVectors.

So we resolve this problem by allowing NullVectors to have distinct field names.

This also makes the NullVector consistent with other types of vectors.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-13645

[kiszk]

LGTM
nit: The last line of the description may have a typo, This also makes the ZeroVector with other types of vectors. looks correct based on the change.

[liyafan82]

LGTM
nit: The last line of the description may have a typo, This also makes the ZeroVector with other types of vectors. looks correct based on the change.

@kiszk Thanks for your feedback. I have updated the description accordingly.

[kiszk]

I still see NullVector instead of ZeroVector. But, it is nit.

[liyafan82]

I still see NullVector instead of ZeroVector. But, it is nit.

@kiszk Thanks for your careful review.
According to the current implementation, ZeroVector is a sub-class of NullVector, and the patch applies to both ZeroVector and NullVector. So I used the more general term (NullVector) here.