DecimalFormat should fail gracefully with empty string currency

The ICU DecimalFormat setCurrency docs says:

* @param theCurrency a 3-letter ISO code indicating new currency
     * to use.  It need not be null-terminated.  May be the empty
     * string or NULL to indicate no currency.

However, my code in ICU 62 was written erroneously assuming that the string is always either nullptr or a valid 3-character code. A test case came up in which a user was calling the API with the empty string, causing a hard-to-debug test failure that didn't always reproduce.

The fix is simple, and I will commit it to this ticket in a few minutes.