Allow me to call GetVariant with context but without a default variant

[thomasheartman]

Is your feature request related to a problem? Please describe.

When I use GetVariant with context (as you need to do when using gradual rollout and a number of other strategies + any constraints), I need to also provide a fallback value (default variant).

Now, I can import Unleash.Internal.Variant.DISABLED_VARIANT, but importing an Internal value feels dodgy. I could potentially create my own variant, but I'm not really interested in that.

All I want to do is to get the correct variant, and if the feature is disabled, then to get the disabled variant.

Describe the solution you'd like

I would like to an override that is GetVariant(featureName, context).
OR I would like GetVariant(featureName, context, null) to not throw a null reference exception. If the end user provides a null value, then the SDK should fall back to the disabled variant. (Is this a bug? 🐛 )