Fix Call to method Assert\AssertionChain::isJsonString() will always evaluate to true.

[EmilMassey]

Fixes this:

function assertJson(string $value): void
{
  Assert::that($value)->isJsonString(); // Call to method Assert\AssertionChain::isJsonString() will always evaluate to true.
}

Also narrows type to non-empty-string, because the assertion fails if empty string is provided.

I based this implementation on phpstan/phpstan-webmozart-assert#143