relevant expression inside repeat with no entries causes error when adding new entries to a different repeat

[jkuester]

Describe the bug

Adding this here for visibility/linkability. Errors have been observed in some forms where no context element found is printed to the console when adding new entries to a repeat when another repeat exists in the same form that contains a field with a relevant expression.

See the Enketo issue (enketo/enketo#1377) for more details and steps to reproduce.

To be clear, this is a problem with Enketo (and reproducible outside of cht-core). The best resolution will involve fixing this in Enekto, though it may be possible to resolve this with a temporary patch in cht-core.