Appends a separator if the builder is currently non-empty. Appending a null separator will have no effect. The separator is appended using {@link #append(String)}.
This method is useful for adding a separator each time around the loop except the first.
for (Iterator it = list.iterator(); it.hasNext(); ) { appendSeparator(","); append(it.next()); }
Note that for this simple example, you should use {@link #appendWithSeparators(Collection,String)}.
@param separator the separator to use, null means no separator
@return this, to enable chaining
@since 2.3