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GH-41136: [C#] Recompute null count for sliced arrays on demand (#41144)

### Rationale for this change

See #41136. This is required for writing sliced arrays to IPC files and streams. It should also be generally useful too by allowing users to be able to rely on valid null count values for sliced arrays.

### What changes are included in this PR?

Adds a new `ArrayData.GetNullCount()` method which computes the actual null count if it is unknown, and changes most uses of `ArrayData.NullCount` to `GetNullCount()`.

### Are these changes tested?

Yes, I've extended the existing slicing tests to also verify the null count, and added new tests specifically for union arrays, which needed special handling.

### Are there any user-facing changes?

Yes, this is a user-facing behaviour change. I don't expect that it should break any existing code as users are unlikely to rely on the NullCount being -1.

* GitHub Issue: #41136

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>