Correctly handle randomizing of Seed.Recursive modifications

[Markoutte]

Description

Fixes #1990

When fuzzer fails to generate any fields of an object, the object was set to null. To fix this now null-this values are ignored. Also, there's a non-optimal logic, when fuzzer try to fuzz values of modified fields first and only then choose a subset of those modifications. With this patch fuzzer choose subset of modifications in Seed.Recursive and only then will fuzz them.

How to test

Automated tests

All fuzzer tests should pass.

Manual tests

Try an example from the issue. It should generate at least 1 trivial test.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.