DefaultValue will return undefined for externref as before, but
for other nullable reference types will return null.

For non-nullable reference types, operations like grow and set
should error if the value to set is missing.

Also adds WPT tests for the error cases. This requires some
updates to wasm-module-builder.js as well.

Fixes issue WebAssembly#501

Co-authored-by: Andreas Rossberg <rossberg@mpi-sws.org>