fix(model): stop Model.validate() crashing on missing nested objects - #16600

Summary

Give a schema a path three levels deep, like profile.address.city, and Model.validate({ name: 'x' }) rejects with TypeError: Cannot read properties of undefined (reading 'address') instead of validating. Model['~standard'].validate() goes through the same code, so it hands that TypeError back as the only issue for input thats perfectly valid.

The loop that throws walks down to each path's parent and never reads what it finds (castObject() does the writing back now), so I've just taken it out.

Examples

Added a test with no profile at all. On master it gets the TypeError, with this it gets the required error for profile.address.city.