fix: Correctly marshal/unmarshal custom types when null/unknown

[amstuta]

Description

Correctly handle Marshalling/Unmarshalling of custom types when they hold a null or unknown value.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (improve existing resource(s) or datasource(s))
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or issues
• I have added acceptance tests that prove my fix is effective or that my feature works
• New and existing acceptance tests pass locally with my changes
• I ran succesfully go mod vendor if I added or modify go.mod file

[deathiop]

Suggested change

	toMarshal := make(map[string]T)
	toMarshal := make(map[string]T, len(t.Elements()))