Avoid warnings for unset array keys

components/article.php

@@ -2,7 +2,7 @@
     <?php if(has_post_thumbnail()) {
         // A header configured specifically for this article
         $url = get_the_post_thumbnail_url();
-    } elseif(!$args["hide_default_header_image"]) {
+    } elseif(!($args["hide_default_header_image"] ?? false)) {
         // Fall back to the default header (which can be configured site-wide, so we can't hardcode the URL)
         $url = get_header_image();
     } else {
@@ -16,14 +16,14 @@
     } ?>
 
     <header <?php echo $style; ?>>
-        <?php if($args["link_title"]): ?>
+        <?php if($args["link_title"] ?? false): ?>
             <h1><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
         <?php else: ?>
             <h1><?php the_title(); ?></h1>
         <?php endif; ?>
     </header>
     <div class="content">
-        <?php if($args["show_only_excerpt"]) {
+        <?php if($args["show_only_excerpt"] ?? false) {
             the_excerpt();
         } else {
             the_content();