Fix KernelVersion comparisons

In the __lt__ function for the class KernelVersions, if
self.groups != other.groups, but self.groups == other.groups[0:n] or
self.groups[0:n] == other.groups for some n, then at some point, one of
the two pieces to be compared will be None, which resulted in an
Exception when calling isdigit.

Hence check whether one of the pieces to be compared is None and handle
this as a special case.

(cherry picked from commit b901203390b4994a8169021d7dc47928561dad24)

qubesmanager/utils.py

@@ -134,6 +134,10 @@ class KernelVersion:  # pylint: disable=too-few-public-methods
                 self.groups, other.groups):
             if self_content == other_content:
                 continue
+            if self_content is None:
+                return True
+            if other_content is None:
+                return False
             if self_content.isdigit() and other_content.isdigit():
                 return int(self_content) < int(other_content)
             return self_content < other_content