Custom Meta Box If Else Statement

Question

You can try this:

<?php  
// get image source:
$metaboxtext3 = esc_attr( get_post_meta( get_the_ID(), 'my_meta_box_text3', true ) );

// get link:
$metaboxtext4 = esc_attr( get_post_meta( get_the_ID(), 'my_meta_box_text4', true ) );

// check if the image source exists:
if(strlen($metaboxtext3)>0){ ?>
   <a href="https://wordpress.stackexchange.com/questions/84788/<?php echo $metaboxtext4; ?>"><img src="<?php echo $metaboxtext3; ?>" alt="" onerror="this.style.display='none'"/></a> 
<?php } ?>