arrow

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

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.

Closes #10949 from liyafan82/fly_0817_zr

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: liyafan82 <fan_li_ya@foxmail.com>