correctly check nil stream (#8905)

Fixes: #8893

This PR changes the
`[checkForHealthyStream`](https://github.com/grpc/grpc-go/blob/944f0585ca8664eb8f09635531c3c9d52d1e8bbb/internal/transport/keepalive_test.go#L815)
function in keepalive_test.go to check for nil stream before closing the
stream. This will make sure the error is returned and logged correctly
instead of throwing a panic when the stream creation fails.

RELEASE NOTES: None

Author: eshitachandwani

internal/transport/keepalive_test.go

@@ -816,8 +816,11 @@ func checkForHealthyStream(client *http2Client) error {
 	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
 	defer cancel()
 	stream, err := client.NewStream(ctx, &CallHdr{}, nil)
-	stream.Close(err)
-	return err
+	if err != nil {
+		return err
+	}
+	stream.Close(nil)
+	return nil
 }
 
 func pollForStreamCreationError(client *http2Client) error {