fix: add omitempty to optional nullable fields (oapi-codegen#2221)

Fixes oapi-codegen#2091

The `omitempty` JSON tag was not being added to optional nullable fields.
The condition `!p.Nullable && shouldOmitEmpty` explicitly prevented any
nullable field from receiving `omitempty`, even when the field was
optional (not required). This contradicted the documented behavior.

The fix removes the `!p.Nullable` guard so nullable fields follow the
same `omitempty` rules as non-nullable fields. The special-case
exception for the `nullable-type` output option is no longer needed
since the logic is now uniform.

Note: `x-go-type-skip-optional-pointer: true` on a nullable field
suppresses the pointer (generating `string` instead of `*string`) but
still correctly receives `omitempty` when the field is optional. Whether
skip-optional-pointer should be allowed to suppress the pointer on
nullable fields is a separate concern.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

internal/test/issues/issue-1039/defaultbehaviour/types.gen.go

@@ -117,8 +117,8 @@ func TestExtPropGoTypeSkipOptionalPointer(t *testing.T) {
 	assert.NoError(t, err)
 
 	// Check that optional pointer fields are skipped if requested
-	assert.Contains(t, code, "NullableFieldSkipFalse *string `json:\"nullableFieldSkipFalse\"`")
-	assert.Contains(t, code, "NullableFieldSkipTrue  string  `json:\"nullableFieldSkipTrue\"`")
+	assert.Contains(t, code, "NullableFieldSkipFalse *string `json:\"nullableFieldSkipFalse,omitempty\"`")
+	assert.Contains(t, code, "NullableFieldSkipTrue  string  `json:\"nullableFieldSkipTrue,omitempty\"`")
 	assert.Contains(t, code, "OptionalField          *string `json:\"optionalField,omitempty\"`")
 	assert.Contains(t, code, "OptionalFieldSkipFalse *string `json:\"optionalFieldSkipFalse,omitempty\"`")
 	assert.Contains(t, code, "OptionalFieldSkipTrue  string  `json:\"optionalFieldSkipTrue,omitempty\"`")

internal/test/schemas/schemas.gen.go

@@ -734,11 +734,7 @@ func GenFieldsFromProperties(props []Property) []string {
 		shouldOmitEmpty := (!p.Required || p.ReadOnly || p.WriteOnly) &&
 			(!p.Required || !p.ReadOnly || !globalState.options.Compatibility.DisableRequiredReadOnlyAsPointer)
 
-		omitEmpty := !p.Nullable && shouldOmitEmpty
-
-		if p.Nullable && globalState.options.OutputOptions.NullableType {
-			omitEmpty = shouldOmitEmpty
-		}
+		omitEmpty := shouldOmitEmpty
 
 		omitZero := false