Repository get_instances silently uses the system context if you pass the wrong thing

Steps to reproduce:

1. Accidentally write the code

$repositories = repository::get_instances(['currentcontext' => $context->id]);

Actual result: it seems to work, until some time later things mysteriously don't work and you have to spend a lot of time debugging it.

Expected result (once you realise it is buggy): A debugging notice like "'currentcontext' passed to repository::get_instances was not a context object. Using system context instead, but you should probably fix your code."