fix(transform): fix nil returned by TransformString

[Roytangrb]

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.

In next TranformString, while checking if if value is zero at

survey/validate.go

Line 86 in e64d9b6

return reflect.DeepEqual(v.Interface(), reflect.Zero(v.Type()).Interface())

reflect.ValueOf(nil).Interface() and reflect.Zero(reflect.ValueOf(nil).Type()).Interface()will cause program to panic

To fix:
return zero value of string ("") instead coz String Tranformer is guaranteed to return string

fix #329

[AlecAivazis]

Nice catch! Thanks for submitting a fix