Fix fill wrapping when text starts with whitespace

[karl]

There is a bug in the fill implementation in #1120 when text starts with a whitespace.

Input:

leading_whitespace =
  <div> First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth</div>

Output on master:

leading_whitespace = (
  <div>
    {" "}
    First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh Twelfth Thirteenth Fourteenth
  </div>
);

The fill algorithm expects to be passed alternating content and whitespace parts, starting with content. Before this PR it was possible for it to start with a whitespace, throwing off the algorithm.

Output with this PR:

leading_whitespace = (
  <div>
    {" "}
    First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh
    Twelfth Thirteenth Fourteenth
  </div>
);

[vjeux]

Thanks :)