Fixed None is not a string in lino.core.diff
Fixed None is not a string in lino.core.diff

lsaffre/lino,lsaffre/lino,lino-framework/lino,lsaffre/lino,lino-framework/lino,lsaffre/lino,khchine5/lino,lino-framework/lino,lsaffre/lino,khchine5/lino,khchine5/lino,khchine5/lino,khchine5/lino,lino-framework/lino,lino-framework/lino