Emit "" when converting implicit <td>s to text in TokenStreamPatcher

* This, in combination with the previous patch, fixes the off-by-1
  DSR error causing selser corruption on the page reported in
  bug 63779.

* Updated existing parser test to catch regressions for this fix.

Bug: 63779
Change-Id: Ied172a9d7e2e3efa1ce22e62e02cfb89fbb3c9dd