Link error in Null VM-hosting binaries with _main_void symbol defined

[leonm1]

#500 introduced a new function signature () -> Word / WasmCallWord<0>, but this overload of NullPlugin::getFunction is not implemented.

[PiotrSikora]

Right! I recall that NullVM fallouts were the primary reason why we always opened PRs against Envoy before merging...

Thanks for catching it! Should be fixed in #517.