Why is get_post_format() for “Standard” returns empty

Because “Standard” is not a format itself – it simply implies that the post has no formats.

…which is somewhat deceptive, given it’s listed in the Format meta-box as a format to choose

I think “standard” quite clearly implies what it means. If you’re creating a site in which a post is never “standard”, then fallback to a default:

if ( ! $format = get_post_format() )
    $format="gallery";

Alternatively, hook onto wp_insert_post and enforce a format if one has not already been set.

function wpse_58121_set_default_format( $post_id ) {
    if ( ! get_post_format( $post_id ) )
        set_post_format( $post_id, 'gallery' );
}