code: public <T, E extends Exception> Nullable<T> mapIfNotEmpty(Try.Function<? super String, T, E> mapper) throws E {
          N.checkArgNotNull(mapper);
  
          return buffer == null ? Nullable.<T> empty() : Nullable.of(mapper.apply(toString()));
      }
docs: Call {@code mapper} only if at least one element/object/entry is appended.
  
  <pre>
  The underline {@code StringBuilder} will be recycled after this method is called if {@code resueStringBuilder} is set to {@code true},
  and should not continue to this instance.
  </pre>
  
  @param mapper
  @return