[#130] Support for Optional Values

[yakovmanshin]

[Closes #130]

• Previously, feature-flag stores returned nil when the value was not found
• This behavior made it impossible to distinguish between the absence of a value and a literal nil value
• While this distinction wasn’t important in the majority of cases, it could be sometimes
• For that reason, optional values were not supported in YMFF
• Support error throwing in feature-flag stores #132 / [#132] Error Throwing in Feature-Flag Stores #139 changed the protocols for feature-flag stores so they throw errors instead of returning nil for nonexistent values
• This change made it possible to accept nil as a valid feature-flag value