is_from_hfss(): fix TypeError when comments is None
This fixes:
[...]/skrf/io/touchstone.py in is_from_hfss(self)
417         '''
418         status = False
> 419         if 'exported from hfss' in str.lower(self.comments):
420             status = True
421         return status
TypeError: descriptor 'lower' requires a 'str' object but received a 'NoneType'
... when comments is None. Return False instead.
@@ -415,10 +415,13 @@ class Touchstone:
True if the Touchstone file has been produced by HFSS
False otherwise
'''
- status = False
+ if self.comments is None:
+ return False
+
if 'exported from hfss' in str.lower(self.comments):
- status = True
- return status
+ return True
+
+ return False
def get_gamma_z0(self):
'''
