packaging-check-invariants: Avoid crashing on OBS-only packages

If a package only appears on OBS the dashboard fails with an exception:

    Traceback (most recent call last):
      File "/builds/-RogsxNE/0/infrastructure/dashboard/./bin/packaging-check-invariants", line 854, in <module>
        checker.check_git_and_obs_versions(package)
      File "/builds/-RogsxNE/0/infrastructure/dashboard/./bin/packaging-check-invariants", line 238, in check_git_and_obs_versions
        if package.git.default_branch:
           ^^^^^^^^^^^
      File "/builds/-RogsxNE/0/infrastructure/dashboard/./bin/packaging-check-invariants", line 79, in __getattr__
        item = super(DictWrapper, self).__getitem__(attr)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    KeyError: 'git'

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

bin/packaging-check-invariants

@@ -235,7 +235,7 @@ class InvariantChecker:
         if not set(package).intersection({"git", "obs", "published"}):
             return
         if "published" not in package:
-            if package.git.default_branch:
+            if "git" in package and package.git.default_branch:
                 default_branch = package.git.default_branch
                 channels = set(package.git.branches).intersection(self.data.channels)
                 active_channels = [