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ARROW-15960: [C++] Fix crash on adaptive int builder edge cases

AdaptiveIntBuilder (and AdaptiveUIntBuilder) would crash when asked to append 0 nulls to a 0-length builder.

Also fixes the following PyArrow crash:
```python
pa.array([None], pa.list_(pa.dictionary(pa.int32(), pa.string()), 0))
```

Closes #12690 from pitrou/ARROW-15960-adaptive-crash

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