Empty userId breaks gradual rollout 100% expected behavior.

[karimkod]

Describe the bug
When passing an empty userId (string.Empty) the toggle is not enabled. Although the toggle have a gradual rollout strategy of 100%.
When passing a null or a non-empty value, the same toogle is enabled as expected.

To Reproduce
Steps to reproduce the behavior:
Create a toggle with a gradual rollout strategy.
Pass the userId as string.empty ("") :

  var unleashEnabled = unleash.IsEnabled("togglename", new UnleashContext()
  {
      
      UserId = string.Empty

  });

Expected behavior
The value of unleashedEnabled should be always true, since the gradual rollout is 100% and there is not constraints.
Screenshots
If applicable, add screenshots to help explain your problem.

**Version **

    <PackageReference Include="Unleash.Client" Version="4.1.8" />

Additional context
Did my own digging and I think that the source is the calculation of stickinessId https://github.com/Unleash/unleash-client-dotnet/blob/main/src/Unleash/Strategies/FlexibleRolloutStrategy.cs#L43
When userId is empty, it is returned as the first choice of stickinessId but then we do the if (!string.IsNullOrEmpty(stickinessId)) which will be false, always.

[karimkod]

If that's a real issue, I'm ready to open a PR, thanks :)
Need to be iso with other sdks behavior.
Checked node sdk, it doesn't exclude empty userId: https://github.com/Unleash/unleash-client-node/blob/main/src/strategy/flexible-rollout-strategy.ts

[sighphyre]

Hey @karimkod,

Thanks for reporting this! Yes, this appears to be a bug. Empty string should not be doing this!

Oh if you're willing to submit a PR that would be amazing!

[karimkod]

@sighphyre here it is #216