added a check before submitting todo to handle empty case

[shahdelwan0]

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[chrisdavidmills]

Sorry, @shahdelwan0, but we can't just make changes to the example code without also updating the accompanying tutorials.

I am also not convinced this code is the best way to handle this — the preventDefault() should happen regardless of whether the input was empty (otherwise the form will submit as per standard behavior), you've used equality rather than strict equality, and I don't know why the commented out console.logs are necessary to include.