Output field value only if another field has some value

I’m afraid this can’t be done from Kirki… You’d have to custom-code something in your theme.

Checking the value of a color’s opacity however is the easy part.
Kirki already includes the ariColor library (see https://aristath.github.io/ariColor/) so you could get the opacity like this:

$value = get_theme_mod( 'my_setting', '#000000' );
$color = ariColor::newColor( $value, 'auto' );
$opacity = $color->alpha;