NullBufferBuilder::allocated_size Returns Size in Bits

[tustvold]

Describe the bug

As documented in #7089, NullBufferBuilder::allocated_size returns the size in bits. Given no reasonable allocator allocates memory in bits, it is somewhat expected that the quantity returned is in bytes, and returning in bits is a bug.

To Reproduce

Expected behavior

The value should be in bytes.

Additional context

This was added in #6047 by @XiangpengHao

[shuozel]

It seems the NullBufferBuilder is calling the BooleanBufferBuilder.capacity() here 1, and we are directly returning the capacity in bits returend by the BooleanBufferBuilder.

Just wanna confirm, we should only make the change to the NullBufferBuilder.capacity() to return the capacity in bytes and leave the BooleanBufferBuilder.capacity() unchanged to return the capacity in bits. Please correct me if my understanding is incorrect.

[alamb]

label_issue.py automatically added labels {'arrow'} from #7104

[alamb]

label_issue.py automatically added labels {'documentation'} from #7089