Evolver had a couple of issues during app evolution that could break

when evolving only part of a database or evolving an app that doesn't

contain models.
An assumption was made that the database signature would contain an app

signature for every app in the recorded migration list, but this wasn't

guaranteed to be the case. It probably would be in practice, but some

upcoming unit test work triggered a breakage here. We now conditionally

check if the app signature is known before trying to set anything on it.
If an app had evolutions but no models (which could be used for changing

app labels), and that app was new in the signature, then the list of

recorded evolutions would remain blank, causing future issues. Some

logic was nested at the wrong level, making the evolution list dependent

on the model list.
This change fixes both issues.

All unit tests pass.