Slight optimization in codegen

[jeffcharles]

Description of the change

Wrap the WasiCtx reinitialization in a check so the context is only initialized when it's set to None on the store. This will only initialize it once as opposed to every time.

Why am I making this change?

This will be slightly faster.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.