Update lib/Pod/PseudoPod/DOM.pm

Fixed problems parsing =headrow

* DOM Before Patch *

Table
  TableRow (headrow)
    Text::Plain (emtpy)
  TableRow (row)
    TableCell  

* DOM After Patch *

Table
  TableRow (headrow)
    TableCell

Before died in the "fixup" method for TableRow for the headrow with the empty Text::Plain