Improve type checking in editor metabox class

[benlk]

developer-driven-custom-post-classes/includes/class-editor-metabox.php

Line 146 in cde8313

if ( !is_array( $meta[0] ) || empty( $meta[0] ) ) {

With WP_DEBUG, this causes output like:

Notice: Uninitialized string offset: 0 in /mstoday/wp-content/plugins/developer-driven-custom-post-classes/includes/class-editor-metabox.php on line 146
class="clearfix post-697619 post type-post status-publish format-standard has-post-thumbnail hentry category-coronavirus-in-mississippi prominence-homepage-featured prominence-top-story featured-media featured-media-image">