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