tpl/transform: Don't fail on "no data to transform"

Fixes #12964

Author: bep

tpl/transform/unmarshal.go

@@ -112,9 +112,10 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 	if err != nil {
 		return nil, fmt.Errorf("type %T not supported", data)
 	}
+	dataStr = strings.TrimSpace(dataStr)
 
 	if dataStr == "" {
-		return nil, errors.New("no data to transform")
+		return nil, nil
 	}
 
 	key := hashing.MD5FromStringHexEncoded(dataStr)

tpl/transform/unmarshal_test.go

@@ -139,6 +139,8 @@ func TestUnmarshal(t *testing.T) {
 a;b;c`, mime: media.Builtin.CSVType}, map[string]any{"DElimiter": ";", "Comment": "%"}, func(r [][]string) {
 			b.Assert([][]string{{"a", "b", "c"}}, qt.DeepEquals, r)
 		}},
+		{``, nil, nil},
+		{`   `, nil, nil},
 		// errors
 		{"thisisnotavaliddataformat", nil, false},
 		{testContentResource{key: "r1", content: `invalid&toml"`, mime: media.Builtin.TOMLType}, nil, false},