Allow Unit for ResultRecipient

[margarita-v]

Currently, it's impossible to use Unit as a result type for ResultRecipient according to InitialValidator.validateResultType, though would be useful as a sign of completed work without data.

UPD I've tried to achieve it (simply extend primitives list for InitialValidator.validateResultType), but it seems to be a restriction of SavedStateHandle.ACCEPTABLE_CLASSES:

    java.lang.IllegalArgumentException: Can't put value with type class kotlin.Unit into saved state
        at androidx.lifecycle.SavedStateHandle.validateValue(SavedStateHandle.java:271)
        at androidx.lifecycle.SavedStateHandle.set(SavedStateHandle.java:251)
        at com.ramcosta.composedestinations.result.ResultBackNavigatorImpl.setResult(ResultBackNavigatorImpl.kt:30)