Add note to GPG key response if user has no keys (#18961)

* Prevent invalid key response.

* Display note instead of 404 response.

* Fixed test.

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

integrations/user_test.go

@@ -121,6 +121,7 @@ func TestExportUserGPGKeys(t *testing.T) {
 	defer prepareTestEnv(t)()
 	// Export empty key list
 	testExportUserGPGKeys(t, "user1", `-----BEGIN PGP PUBLIC KEY BLOCK-----
+Note: This user hasn't uploaded any GPG keys.
 
 
 =twTO