lint(validators): refactor len conditions

[ee7]

In our validation procs, there are conditions like

if data.hasKey(key):
  if data[key].kind == JString:

where the condition being true represents the "happy path".

To improve readability, let's try to do this consistently for every
condition apart from the innermost one; it's easier to see the "happy
path" when it minimises the number of else.

This commit refactors the len conditions accordingly, so that we
prefer len > 0 and use len == 0 only when there is no else.

This commit also changes a var to a let to better convey that we
don't mutate.

[ErikSchierboom]

LGTM

[ee7]

I've again verified that this commit keeps the output of configlet lint the same on every track. I intend to add that to CI eventually, although it can't be a "test" exactly - some commits will be expected to change the output of configlet lint.