Add more asserts to check matrix validity (#31701)

## Description

These will help identify where the matrix starts to get wrong. 

Also fixed `RenderFittexBox` to no longer paint with empty child which previously triggered invalid matrix computations (NaN with dividing by 0). See also https://github.com/flutter/flutter/pull/7489

## Related Issues

https://github.com/flutter/flutter/issues/31650
https://github.com/flutter/flutter/issues/31700
https://github.com/flutter/flutter/issues/7431

## Tests

* RenderFittedBox does not paint with empty sizes