Fix a DecimalFormat crash when formatting the empty string.

We don't even need a general-purpose iterator, and it's never exposed
to user code, so we can simplify things quite a bit. The key change
though is that a null array is not an error condition --- it just
means we didn't actually output anything.

Bug: https://code.google.com/p/android/issues/detail?id=59600
Change-Id: I7ec0bab00e15ab5c3a4dbd15e2dc81494f68e82f