Add default dispose callback

[sparksp]

I was seeing console errors due to a null myDisposeCallback. Nowhere seems to be calling module.hot.dispose but module.hot.apply is still called. There should probably be some type check that the callback is callable but to keep the code clean I have provided a simple noop callback instead.

Here's the config that was failing for me:

elm-live resources/elm/Main.elm \
    --hot --dir=public --start-page=index.html \
    --proxy-prefix=/api --proxy-host=http://localhost:8088/api \
    -- --output=public/js/main.js --debug

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