function hasNullEntry(entry, tableName) {
  var keys = Object.keys(entry.row.payload()[tableName]);
  assertTrue(keys.length > 0);
  return Object.keys(entry.row.payload()[tableName]).every(function(key) {
    return entry.row.payload()[tableName][key] === null;
  });
}
Checks that the given combined entry has a null entry for table 'tableName'.
@param {!lf.proc.RelationEntry} entry The combined entry.
@param {!string} tableName
@return {boolean}