I just realized this:
1. Enable "Enable Soundcloud Upload" in preference and fill in the username+password, click save
2. Go back to preference, change unrelated setting like "Station Name", click save
3. Now since soundcloud password field is empty (expected, as we don't want others to see how many digits
are there), the save failed
This commit is to fix this. Created another custom validator for the password field, which does the same
thing as ConditionalNotEmpty validator but allow password field to be empty when the username/email field
wasn't changed.
Yuchen Wang