Refactor PropertyResolverGenerator to handle nullable properties and update tests for resolver generation

[tombiddulph]

This pull request enhances the property resolver generator to correctly handle nullable properties and improves the generated code's handling of these cases. It also updates and expands the test suite to ensure that resolvers are always generated (even if empty) and that nullable and non-nullable properties are handled as expected.

Generator improvements:

• The generator now detects whether a property is nullable (either nullable value type or nullable reference type) and uses the null-conditional operator (?.ToString()) in the generated resolver code for such properties. Non-nullable properties use the standard .ToString() call. [1] [2] [3] [4] [5] [6]

Test updates and additions:

• Tests are updated to expect that resolvers are always generated, even when there are no matching or only generic types, and to verify that the generated code is empty in those cases. [1] [2] [3] [4]
• New tests verify that nullable reference and value type properties use the null-conditional operator in the generated code, while non-nullable properties do not.

Code modernization:

• Uses C# collection expressions for constructing arrays more concisely in resolver config parsing.