fix: remove placeholder for custom css in number input

Author: Arukuen

src/components/advanced-range-control/range-control.js

@@ -144,6 +144,11 @@ const StackableRangeControl = memo( props => {
 		placeholderValue = initialPosition
 	}
 
+	// Remove placeholder if it's a custom CSS
+	if ( typeof placeholderValue === 'string' && placeholderValue.startsWith( 'var' ) ) {
+		placeholderValue = ''
+	}
+
 	return <div
 		className={ classNames }
 		style={ { '--ugb-advanced-range-control--width': percentageValue } }