scripts: avoid error in DCO check on empty branches

If the DCO check is run on an empty branch (ie one which has no commits
different from master), it throws an error due to trying to interpret
the empty string as a git commit SHA.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

scripts/require-dco.py

@@ -46,7 +46,10 @@ print("\nChecking for 'Signed-off-by: NAME <EMAIL>' on all commits since %s...\n
 log = subprocess.check_output(["git", "log", "--format=%H %s", ancestor + "..."],
                               universal_newlines=True)
 
-commits = [[c[0:40], c[41:]] for c in log.strip().split("\n")]
+if log == "":
+    commits = []
+else:
+    commits = [[c[0:40], c[41:]] for c in log.strip().split("\n")]
 
 for sha, subject in commits: