Fixed crash when exp(0) of p-adic numbers is called

[Noel-Roemmele]

Fixes #38037. Fixes crash when exp(0) is called when using p-adic numbers. Since the unit part of 0 does not exist it was causing a crash. Therefore, to fix this bug we check if our exponent is exactly 0 and if it is return 1 immediately. Also added a doctest to verify that the issue has been fixed.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[DaveWitteMorris]

LGTM. I will set to positive review on Wednesday if there are no other comments and make ptestlong (or CI testing) shows no problems.

[DaveWitteMorris]

Oh, I missed this -- you do need to make one minor fix: There should only be one colon at the end of TESTS::, so you need to delete one of them. (Two colons are used when the following block is a code block, but you have a text comment before the code block.)

[DaveWitteMorris]

make ptestlong came back ok, so you should just need to delete the extra colon mentioned above.

[DaveWitteMorris]

Sorry, you need to make another minor correction in line 2964: change :trac: to :issue:.