Preserve Traits nil value between proto<-->API type conversion (#44492)

api/types/trait/convert/v1/trait.go

@@ -25,15 +25,21 @@ import (
 
 // FromProto converts an array of v1 traits into a map of string to string array.
 func FromProto(traits []*traitv1.Trait) trait.Traits {
+	if traits == nil {
+		return nil
+	}
 	out := map[string][]string{}
 	for _, trait := range traits {
 		out[trait.Key] = trait.Values
 	}
 	return out
 }
 
-// ToV1 converts a map of string to string array to an array of v1 traits.
+// ToProto converts a map of string to string array to an array of v1 traits.
 func ToProto(traits trait.Traits) []*traitv1.Trait {
+	if traits == nil {
+		return nil
+	}
 	out := make([]*traitv1.Trait, 0, len(traits))
 	sortedKeys := make([]string, 0, len(traits))
 

api/types/trait/convert/v1/trait_test.go

@@ -36,3 +36,8 @@ func TestRoundtrip(t *testing.T) {
 
 	require.Empty(t, cmp.Diff(traits, converted))
 }
+
+func TestNil(t *testing.T) {
+	var traits trait.Traits
+	require.Nil(t, FromProto(ToProto(traits)))
+}