arrow

GH-41741: [C++] Check that extension metadata key is present before attempting to delete it (#41763)

### Rationale for this change

Neither Schema.fbs nor the Arrow C Data interface nor the columnar specification indicates that the ARROW:extension:metadata key must be present; however, the `ImportType()` implementation assumes that both `ARROW:extension:name` and `ARROW:extension:metadata` are both present and throws an exception if `ARROW:extension:metadata` is missing. This causes pyarrow to crash (see issue for reproducer).

### What changes are included in this PR?

This PR checks that the extension metadata is present before attempting to delete it.

### Are these changes tested?

Yes (test added).

### Are there any user-facing changes?

No.
* GitHub Issue: #41741

Authored-by: Dewey Dunnington <dewey@voltrondata.com>
Signed-off-by: Dewey Dunnington <dewey@voltrondata.com>