Fix return value of hook calls that have no handlers.

For this case, the return value is always true.

src/Func.cc

@@ -288,7 +288,7 @@ Val* BroFunc::Call(val_list* args, Frame* parent) const
 		assert(Flavor() == FUNC_FLAVOR_EVENT || Flavor() == FUNC_FLAVOR_HOOK);
 		loop_over_list(*args, i)
 			Unref((*args)[i]);
-		return 0 ;
+		return Flavor() == FUNC_FLAVOR_HOOK ? new Val(true, TYPE_BOOL) : 0;
 		}
 
 	SegmentProfiler(segment_logger, location);

testing/btest/Baseline/language.hook/out

@@ -3,6 +3,7 @@ myhook return F
 myhook return T
 myhook, &priority=5, [a=37, b=goobye world]
 F
+T
 myhook3, 8
 T
 myhook4, 2