add fallback image for jumbotron featured image

When Jumbotron is set to use featured images, it would be very helpful to fall back to the default image from home.

Use case: I set “Featured image as jumbotron” in customizer and then forget to set a featured image for a post/page. Currently I get a white background, which is unpleasant, as the jumbotron is also white.

Solution: Simply add

} else {
		$style = 'background-image: url(' . esc_url( $jumbotron_general_image ) . ');';
	}

in header.php on line 29.