Warn when no downstream targets are defined

comma/config.py

@@ -68,7 +68,7 @@ class FullConfig(BasicConfig):
 
     repos: Dict[str, AnyUrl]
     upstream: Upstream
-    downstream: Optional[Tuple[Target, ...]]
+    downstream: Optional[Tuple[Target, ...]] = ()
     spreadsheet: Optional[Spreadsheet] = Spreadsheet()
 
     @validator("repos")

comma/downstream/__init__.py

@@ -58,6 +58,9 @@ class Downstream:
             subjects = session.query(MonitoringSubjects).all()
             total = len(subjects)
 
+            if not total:
+                LOGGER.warning("No downstream targets defined")
+
             for num, subject in enumerate(subjects, 1):
                 if subject.distroID.startswith("Debian"):
                     # TODO (Issue 51): Don't skip Debian