Fix omitempty tag for nullable fields

Description

This PR fixes a bug in the JSON tag generation for struct fields that represent nullable but non-required properties in OpenAPI schemas.

The issue was in the conditional logic that determines when to add the omitempty tag:

// Before
shouldOmitEmpty := (!p.Required || p.ReadOnly || p.WriteOnly) &&
    (!p.Required || !p.ReadOnly || !globalState.options.Compatibility.DisableRequiredReadOnlyAsPointer)

omitEmpty := !p.Nullable && shouldOmitEmpty

The bug occurred because !p.Nullable would be false for nullable fields, causing omitEmpty to be false even when the field wasn't required. This led to nullable but optional fields incorrectly missing the omitempty tag.

Changes

• Fixed the conditional logic to correctly apply omitempty to all non-required fields regardless of nullability
• Updated tests to verify the corrected behavior

Why this makes sense

1. According to the OpenAPI spec, nullable: true means a value can be null, while the required array determines if a property must be present
2. In Go, the omitempty tag controls whether a field is omitted when it's the zero value (not directly related to nullability)
3. Non-required fields should have omitempty to correctly handle JSON serialization when the field is absent

This change ensures the generated code better aligns with both OpenAPI semantics and Go's JSON marshaling conventions.