Allow no eval_loader when eval is disabled

[b-chu]

What does this PR do?

Allows no eval_loader to be specified when eval_interval=0 which means eval is disabled

What issue(s) does this change relate to?

GRT-2442

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[mvpatel2000]

Great QoL improvement