fix(input): $render input even if $modelValue is empty

$render should only be concerned if there is a $viewValue present.
When we check $isEmpty($modelValue), we assume $render is used in
context of a model -> view update. But since it is part of the public
API, it needs to work independent of internal usage.

Fixes #9156

In the end, the formatters are responsible for providing a correct value to the $render function in a model -> update. If someone uses $render manually, they are obviously responsible.