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ARROW-7764: [C++] Don't keep a null bitmap in ArrayData if null_count == 0

This spotted a couple bugs in kernels where null bitmaps were always iterated even if null.

Closes #6447 from pitrou/ARROW-7764-optimize-no-nulls and squashes the following commits:

577c265fd <Antoine Pitrou> Remove pointless DecodeArrowNonNull overrides.
ba1e3a4fe <Antoine Pitrou> Fix R failure
5575798ed <Antoine Pitrou> ARROW-7764:  Don't keep a null bitmap in ArrayData if null_count == 0

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>