Return false instead of crashing when asking if a repeat can be added at a deleted position

[grzesiek2010]

Closes #getodk/collect#5539

What has been done to verify that this works as intended?

I've added tests and also tested the fix with ODK Collect.

Why is this the best possible solution? Were any other approaches considered?

In ODK Collect when we remove a repeat we need to find the last relevant index. In this case we navigate back and check every possible index. If something does not exist we don't want any exceptions but just simple false value.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

It's a safe fix so testing the described scenario should be enough.

Do we need any specific form for testing your changes? If so, please attach one.

any form with nested repeats and repeat_count like the one attached in getodk/collect#5539

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

[lognaturel]

I initially was concerned that this change might negatively affect certain cases but I've spent some time trying to come up with an example and I can't. I confirmed that it does fix the issue that was filed.