Fix potential crash when comparing snap versions

When comparing two snap versions that meet the following conditions, a
TypeError was raised:

 * a.major == b.major
 * a.minor == b.minor
 * a.git_build_id is None and b.git_build_id is None

Fixed by first converting the git_build_id of both variables to a float
before comapring them.

Also added a unit-test to cover this scenario.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/common/snap.py

@@ -76,9 +76,9 @@ class SnapVersion:
         elif self.patch < other.patch:
             return False
 
-        if self.git_build_id is not None and other.git_build_id is None:
-            return True
-        elif self.git_build_id is None and other.git_build_id is not None:
-            return False
+        build_id_self = \
+            float("-inf") if self.git_build_id is None else self.git_build_id
+        build_id_other = \
+            float("-inf") if other.git_build_id is None else other.git_build_id
 
-        return self.git_build_id > other.git_build_id
+        return build_id_self > build_id_other

subiquity/common/tests/test_snap.py

@@ -43,3 +43,7 @@ class TestSnapVersion(unittest.TestCase):
         # Make sure we ignore the build ID when the patch version is different
         self.assertLess(SnapVersion.from_string("21.10.2+git255.deadbeef"),
                         SnapVersion.from_string("21.10.3+git135.deadbeef"))
+
+        snap_version = SnapVersion.from_string("21.10.02")
+        self.assertFalse(snap_version < snap_version)
+        self.assertFalse(snap_version > snap_version)