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ARROW-7020: [Java] Fix the bugs when calculating vector hash code

When calculating the hash code for a value in the vector, the validity bit must be taken into account.

Closes #5752 from liyafan82/fly_1029_hash and squashes the following commits:

e5b1ce3d2 <liyafan82>  Reference the same variable for null hash code
30e591f1c <liyafan82>  Fix the problem of inconsistency for UnionVector#isNull
98e2daf91 <liyafan82>  Fix the bugs when calculating vector hash code

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>