Add support for empty input lists

[tvandinther]

Description of your changes

With the changes in this MR, the function now completes successfully with empty input lists or refs which resolve to empty lists.

Added a new optional boolean field on the function input named failOnEmpty. This field opts into the previous behaviour of this function in case it is desired.

Fixes #67

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[ytsarev]

@tvandinther thanks a lot for the PR. It is useful! Could you please add unit tests that cover the logic?

[tvandinther]

@tvandinther thanks a lot for the PR. It is useful! Could you please add unit tests that cover the logic?

The current test suite mocks the whole query function with an implementation which performs the same check as what I have modified in the real implementation. https://github.com/upbound/function-msgraph/blob/main/fn_test.go#L374

Unsure how to proceed exactly since these changes sit below the mocked interface. I considered exiting early but that is technically a different implementation as the target should continue to have (empty) results written and status should continue as usual, rather than the query being skipped altogether.

[tvandinther]

For now I've updated the mocks to have the same changes as in the real implementation to show that these changes work as intended, even though the code is not in the test path.

[ytsarev]

Can you please add example to README and document new configuration field in https://github.com/upbound/function-msgraph?tab=readme-ov-file#input-configuration-options ? Otherwise, I believe it is good to merge 👍

[tvandinther]

Thanks @ytsarev, README update has been pushed.

[ytsarev]

Awesome! @tvandinther thank you so much for the contribution!