Keep "by" from showing up when author is blank

functions.php

@@ -292,14 +292,17 @@ function bw_get_author() {
  * @since Twenty Ten 1.0
  */
 function twentyten_posted_on() {
-	printf( __( '<span class="%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'twentyten' ),
+	printf( __( '<span class="%1$s">Posted on</span> %2$s%3$s', 'twentyten' ),
 		'meta-prep meta-prep-author',
 		sprintf( '<a href="%1$s" rel="bookmark"><time datetime="%2$s" class="entry-date" pubdate>%3$s</time></a>',
 			get_permalink(),
 			esc_attr( get_the_time('c') ),
 			get_the_date()
 		),
-		bw_get_author()
+		(($author = bw_get_author()) ?
+            sprintf( __( ' <span class="meta-sep">by</span> %s', 'twentyten' ), $author ) :
+            ''
+        )
 	);
 }
 endif;

issue.php

@@ -27,7 +27,7 @@
 ?>
 <dt>
 <a href="<?php echo the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyten' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
-by <?php echo bw_get_author(); ?>
+<?php if ($author = bw_get_author()) { ?>by <?php echo $author; } ?>
 </dt>
 <dd><?php echo get_post_meta($post->ID, 'excerpt', true); ?></dd>
 <?php