Hide empty custom field

I’m assuming what you want is to hide the text when meta is empty.
You can put a conditional check for meta before print:

add_action( 'woocommerce_single_product_summary', 'also_available_on_ps4', 38 ); 
function also_available_on_ps4() {
    global $product;
    if ( has_term( 'ps4-games', 'product_cat' ) ) {
        if(get_post_meta( get_the_ID(), 'also_available', true )){
            echo '<b>Also available on:</b>' . get_post_meta( get_the_ID(), 'also_available', true );
        }
    }
}