Assertions needed to catch zero NIDs

*This issue was originally created by [**@neilalexander**](https://github.com/neilalexander) at .*

All NIDs (event, event type, state block, state snapshot, event state key, room, etc) always start from `1`, but there are sometimes cases where bugs cause us to request `0` NIDs. We should put some assertion to catch these bugs as they happen.

No files, tests, or entry points are named. Start by locating the NID request paths for event, event type, state block, state snapshot, event state key, and room identifiers, then determine where zero values can be caught; done means assertions cover the relevant NID requests and tests demonstrate that zero NIDs are rejected.

Written by the indexing model from the issue text.