dashboard/app: prevent staticcheck false positive

Otherwise it complains:

dashboard/app/tree_test.go:1136:5: SA5011(related information):
	this check suggests that the pointer can be nil
	if found == nil {
	   ^
diff --git a/dashboard/app/tree_test.go b/dashboard/app/tree_test.go
index f12799b..e2e5b8f 100644
--- a/dashboard/app/tree_test.go
+++ b/dashboard/app/tree_test.go
@@ -1135,6 +1135,7 @@
 	}
 	if found == nil {
 		ctx.ctx.t.Fatalf("unknown job request: %#v", resp)
+		return // to avoid staticcheck false positive about nil deref
 	}
 	// Figure out what should the result be.
 	result := treeTestOK