fix(transform): fix nil returned by TransformString for ComposeTranfo…

…rmers (AlecAivazis#330)

TranformString should not return nil on zero value, otherwise nil will be passed to next Tranformer if there are more than 1 Transformer in ComposeTransformers.

return zero value of string instead coz String Tranformer is guaranteed to return string

transform.go

@@ -18,18 +18,21 @@ func TransformString(f func(s string) string) Transformer {
 	return func(ans interface{}) interface{} {
 		// if the answer value passed in is the zero value of the appropriate type
 		if isZero(reflect.ValueOf(ans)) {
-			// skip this `Transformer` by returning a nil value.
+			// skip this `Transformer` by returning a zero value of string.
 			// The original answer will be not affected,
 			// see survey.go#L125.
-			return nil
+			// A zero value of string should be returned to be handled by
+			// next Transformer in a composed Tranformer,
+			// see tranform.go#L75
+			return ""
 		}
 
 		// "ans" is never nil here, so we don't have to check that
-		// see survey.go#L97 for more.
+		// see survey.go#L338 for more.
 		// Make sure that the the answer's value was a typeof string.
 		s, ok := ans.(string)
 		if !ok {
-			return nil
+			return ""
 		}
 
 		return f(s)

transform_test.go

@@ -41,4 +41,10 @@ func TestComposeTransformers(t *testing.T) {
 		// the result should be lowercase.
 		t.Errorf("TestComposeTransformers transformer failed to transform the answer to title->lowercase, expected '%s' but got '%s'.", expected, got)
 	}
+
+	var emptyAns string
+	if expected, got := "", transformer(emptyAns); expected != got {
+		// TransformString transformers should be skipped and return zero value string
+		t.Errorf("TestComposeTransformers transformer failed to skip transforming on optional empty input, expected '%s' but got '%s'.", expected, got)
+	}
 }