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

[Narretz]

$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.

[petebacondarwin]

@tbosch - how does this look? You changed it to use $modelValue in Narretz@3e51b84 previously.

[tbosch]

@petebacondarwin looks good at first sight, but can't dig into this... Assigning to you...

[petebacondarwin]

LGTM - merging

[Narretz]

noooo

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[petebacondarwin]

Fixed by #10164