oldhepdata: check for units if qualifier or header is SQRT(S)

[GraemeWatt]

This line gives an exception AttributeError: 'NoneType' object has no attribute 'lower' if a qualifier with name SQRT(S) has no units:

hepdata-converter/hepdata_converter/parsers/oldhepdata_parser.py

Line 540 in 3f0330d

if name.startswith('SQRT(S)') and units.lower() in ('gev'):

Similarly if SQRT(S) is given as the name of a header without units:

hepdata-converter/hepdata_converter/parsers/oldhepdata_parser.py

Line 292 in 3f0330d

if xheader['name'].startswith('SQRT(S)') and xheader['units'].lower() in ('gev'):

An additional condition should be added to these lines to check that the units are present and not None.