Skip defining getter and setter when property not defined in schema

[hubgit]

This PR contains:

A BUGFIX?

Describe the problem you have without this PR

I'm using RxDB in an app that defines only a minimal schema for each object, as there are lots of different types of object stored in the same collection. I've found that calling get(x) on a document, when the property at x is an object or array, calls _defineGetterSetter which tries to get the schema for the property, which is null when there's no schema defined for this path, causing an exception to be thrown.

[pubkey]

Hi @hubgit
thank you for the PR.
Can you add a test-case that fails without this change?
Just to ensure this still works after refactors. Thank you.