fix(engine): conversion from undefined to JSON

[ryutamago]

Context
Related task

Currently converting undefined to JSON panics, which causes jstz kernel to hang. We should throw a JS error Instead of panicking.
** Test **

To locally test, check out my branch locally, replace the following under the [patch.crates-io] with your local path in Cargo.toml

boa_ast = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_engine = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_gc = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_interner = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_macros = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_parser = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }
boa_profiler = { git = "https://github.com/trilitech/boa.git", branch = "sam.finch@/context/expose-instruction-count" }

Tested that kernel doesn't hang in case undefined is passed to Kv.set
Repl:
Before:

After:

[johnyob]

Lgtm -- would you be able to do a PR on jstz that updates the patches