Fix gr.Code not rendering when it has no value
An interactive gr.Code created without an initial value stopped
rendering in 6.19.0: the backend strips the value key from the
config, so the frontend receives undefined, and binding undefined
to the value $bindable prop that gained a fallback in #13526 throws
props_invalid_value during mount, aborting the component render.
Remove the fallback and coalesce to an empty string where the value
is used internally, keeping the frontend typecheck green.
