CLEANUP: sample: remove uneeded check in json validation

CLEANUP: sample: remove uneeded check in json validation

- check functions are never called with a NULL args list, it is always
  an array, so first check can be removed
- the expression parser guarantees that we can't have anything else,
  because we mentioned json converter takes a mandatory string argument.
  Thus test on `ARGT_STR` can be removed as well
- also add breaking line between enum and function declaration

In order to validate it, add a simple json test testing very simple
cases but can be improved in the future:

- default json converter without args
- json converter failing on error (utf8)
- json converter with error being removed (utf8s)

Signed-off-by: William Dauchy <wdauchy@gmail.com>