fix: IgnoreNullValues regression from MapToTarget cases in #905

[DocSvartz]

#858 Bug overview:
Nullable custom ValueType not mapping to custom ValueType
or NotNullable custom ValueType on a potentially nullable path: property<Class>.property<ValueType>

Causes of occurrence:

1. Not adapter for custom Nullable ValueType
2. ApplyNullPropagation algoritm

If custom ValueType on a potentially nullable path: property<Class>.property<ValueType> ApplyNullPropagation tranform it to (Nullable<ValueType>) property<Class>.property<ValueType>
What is actually required only in the case of MapToTarget is to support IgnoreNullValues feature

Since an initially NotNullable source becomes nullable and can be skipped when the value is null

before ApplyNullPropagation
Expression (property<Class>.property<ValueType>) return ValueType data - not null

after ApplyNullPropagation
Expression ((Nullable<ValueType>) property<Class>.property<ValueType>) return Nullable<ValueType> data - can be null