aws/endpoints: Fix testing bug in partitions other than aws (#2920)

TestEndpointFor_EmptyRegion breaks due to an unknown service called by the test. The fix runs this test only for aws partitions, and skips others.

aws/endpoints/v3model_test.go

@@ -542,6 +542,11 @@ func TestEndpointFor_RegionalFlag(t *testing.T) {
 }
 
 func TestEndpointFor_EmptyRegion(t *testing.T) {
+	// skip this test for partitions outside `aws` partition
+	if DefaultPartitions()[0].id != "aws" {
+		t.Skip()
+	}
+
 	cases := map[string]struct {
 		Service    string
 		Region     string