Support writing IPAC-style tables that contain no data

This fixes the following error:

    >>> import astropy.table
    >>> t = astropy.table.Table(names=['a', 'b'])
    >>> t.write('test.dat', format='ascii.ipac')
    ValueError: max() arg is an empty sequence
Escape backslashes in expected output
Merge pull request #2498 from lpsinger/write_no_data_ipac

Support writing IPAC-style tables that contain no data