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GH-38039: [C++][Parquet] Fix segfault getting compression level for a Parquet column (#38025)

### Rationale for this change

After the changes in #35886, getting the compression level for a Parquet column segfaults if the compression level or other options weren't previously set

### What changes are included in this PR?

Adds a null check on the codec options of the column  properties before trying to access the compression level.

### Are these changes tested?

Yes, I added a unit test.

### Are there any user-facing changes?

This fixes a regression added after 13.0.0 so isn't a user-facing fix
* Closes: #38039

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>