Avoid crash when no services are defined

If no services are defined, podman-compose
crashes as services variable is None,
while the expected behavior might be
an error or the same as docker-compose, which will continue the
execution of the compose file (eg. creating networks, etc).

This commit fixes the crash and allows the program to continue, mimicking
docker-compose behavior.

podman_compose.py

@@ -828,6 +828,10 @@ class PodmanCompose:
             print(" ** merged:\n", json.dumps(compose, indent = 2))
         ver = compose.get('version')
         services = compose.get('services')
+        if services is None:
+            services = {}
+            print("WARNING: No services defined")
+		
         # NOTE: maybe add "extends.service" to _deps at this stage
         flat_deps(services, with_extends=True)
         service_names = sorted([ (len(srv["_deps"]), name) for name, srv in services.items() ])