[sample.fizzbuzz] Fix null error and work around rendering bug

[eNotchy]

• The app displayed a null error message during the first second after startup. Providing :default values to the :watch directives fixed this.
• :sub/opts map deleted because my understanding is that this functionality was removed when cljd.flutter came out of alpha status
• (on Linux with a font size of 64) the number was rendered 1 or 2 pixels higher when "Fizz" and/or "Buzz" is rendered. Using a single space instead of an empty string as the placeholders fixes this.

[dupuchba]

thanks a lot @eNotchy 🙇‍♂️