SetAccess: fix 500 error when no parameter force in access api

The parameter `force` is optional, if it is not present, it should be set
to false by default.

Change-Id: Ia1f787edfe60680b49798eb72220c6de13aee586

gerrit-server/src/main/java/com/google/gerrit/server/project/SetAccess.java

@@ -267,7 +267,9 @@ public class SetAccess implements
               r.setMin(pri.min);
             }
             r.setAction(GetAccess.ACTION_TYPE.inverse().get(pri.action));
-            r.setForce(pri.force);
+            if (pri.force != null) {
+              r.setForce(pri.force);
+            }
           }
           p.add(r);
         }