Correct asserts for fields in deepEquals

[EgorkaKulikov]

Description

This PR fixes two bugs with UtNullModel processing in deepEquals.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Regression and integration tests

Test coverage for deepEquals method

Manual Scenario

Run ContestEstimator with the following settings:

methodFilter = "com.google.common.collect.FilteredEntryMultimap.createAsMap" 
projectFilter = listOf("guava")

Checklist (remove irrelevant options):

This is the author self-check list

• [+] The change followed the style guidelines of the UTBot project
• [+] Self-review of the code is passed
• [+] The change contains enough commentaries, particularly in hard-to-understand areas
• [+] New documentation is provided or existed one is altered
• [+] No new warnings
• New tests have been added
• All tests pass locally with my changes