dns: test empty DNS record IDs

[favonia]

Description

1. Check whether DNS record IDs are empty in DeleteDNSRecord (my oversight in dns: UpdateDNSRecord: clean up params and drop record lookup #1170)
2. Add test cases for empty DNS record IDs for GetDNSRecord, UpdateDNSRecord, and DeleteDNSRecord
3. Use assert/require.ErrorIs to compare errors during testing

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v0.59.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!