derivedFrom: don't LOG(ERROR) if the ownership set is empty

Summary:
This can happen if we didn't record ownership for the base facts in
the DB, it's not an error necessarily.

Reviewed By: josefs

Differential Revision: D51254522

fbshipit-source-id: a664a36e47d046055d37887c9bbb31b241f74c98

glean/rts/ownership/derived.cpp

@@ -19,7 +19,6 @@ namespace rts {
 
 void DefineOwnership::derivedFrom(Pid pid, Id id, const std::set<UsetId>& deps) {
   if (deps.size() == 0) {
-    LOG(ERROR) << "DefineOwnership::derivedFrom: empty deps";
     return;
   }
 
@@ -264,8 +263,10 @@ void addDerived(
       }
     }
 
-    for (auto id : facts) {
-      define->derivedFrom(pid, id, owners);
+    if (owners.size() > 0) {
+      for (auto id : facts) {
+        define->derivedFrom(pid, id, owners);
+      }
     }
   }
 }