Fix bug for symboltable get on empty
Error in WebAssembly type analysis: mishandling of function tables in the case where they are definitely empty, UpperBoundSymbolTable returns a NoneSome(Powerset()) instead of simply None(), which is then not handled properly by Sturdy, which tries to Join the output of all the returned functions by the function table, leading to an empty Join and an uncaught IllegalArgumentException in the joinFold of the EffectStack.