config: Allow empty config files

Change-Id: Ic010e06220421df00405c5b6f5ad2e83514b6dcd
diff --git a/src/osmo_gsm_tester/core/resource.py b/src/osmo_gsm_tester/core/resource.py
index af05a51..62bba07 100644
--- a/src/osmo_gsm_tester/core/resource.py
+++ b/src/osmo_gsm_tester/core/resource.py
@@ -54,7 +54,7 @@
         self.read_conf()
 
     def read_conf(self):
-        self.all_resources = Resources(config.read(self.config_path, schema.get_resources_schema()))
+        self.all_resources = Resources(config.read(self.config_path, schema.get_resources_schema()) or {})
         self.all_resources.set_hashes()
 
     def reserve(self, origin, want, modifiers):