Require bundled product input if it only contains optional products.

functions.php

add_filter( 'woocommerce_bundle_requires_input', 'wc_ninja_require_bundle_input', 10, 2 );
function wc_ninja_require_bundle_input( $requires_input, $product ) {
	if ( ! $product->contains( 'mandatory' ) && $product->contains( 'optional' )  ) {
		$requires_input = true;
	}

	return $requires_input;
}