Passing 'undefined' to 'doAction' should throw error

I propose that `flux.doAction(undefined)` should throw an error. This gives the developer a faster debugging feedback cycle by getting an error immediately whenever an invalid value is passed as an action name. Thoughts?

Start by locating the doAction entry point in fluxify and inspect how action names are currently handled. Confirm the expected behavior for an undefined action value, then run the existing test suite to verify that the error is raised without changing valid action handling.

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