Fix contentRatingDescription never being empty
Various parts of the QML check to see if the string returned by this function is empty, but it never is because as long as you have a new enough AppStream version, the returned value will always at least have an asterisk and a newline.
This commit fixes that problem by generating the final string in the loop, rather than at the end in the return statement. This allows for it to remain empty if nothing happens in the loop.