Issue #841: Empty string return in audio_playlist().

Inside a conditional, return an empty string.
As Weston mentioned, the PHP DocBlock
indicates a string return value.
Also, add an assertion for this.
And correct the documentation of 'content_width.'

includes/embeds/class-amp-playlist-embed-handler.php

@@ -118,11 +118,11 @@ public function shortcode( $attr ) {
 	/**
 	 * Gets an AMP-compliant audio playlist.
 	 *
-	 * @return string Playlist shortcode markup.
+	 * @return string Playlist shortcode markup, or an empty string.
 	 */
 	public function audio_playlist() {
 		if ( ! isset( $this->data['tracks'] ) ) {
-			return;
+			return '';
 		}
 		$container_id   = 'ampPlaylistCarousel' . self::$playlist_id;
 		$selected_slide = $container_id . '.selectedSlide';
@@ -164,7 +164,7 @@ public function audio_playlist() {
 	 * This uses similar markup to the native playlist shortcode output.
 	 * So the styles from wp-mediaelement.min.css will apply to it.
 	 *
-	 * @global content_width.
+	 * @global int content_width.
 	 * @return string $video_playlist Markup for the video playlist.
 	 */
 	public function video_playlist() {

tests/test-class-amp-playlist-embed-handler.php

@@ -185,9 +185,12 @@ public function test_get_thumb_dimensions() {
 	 */
 	public function test_audio_playlist() {
 		$attr                 = $this->get_attributes( 'audio' );
-		$this->instance->data = $this->instance->get_data( $attr );
+		$this->instance->data = array();
 		$playlist             = $this->instance->audio_playlist();
+		$this->assertEquals( '', $playlist );
 
+		$this->instance->data = $this->instance->get_data( $attr );
+		$playlist             = $this->instance->audio_playlist();
 		$this->assertContains( '<amp-carousel', $playlist );
 		$this->assertContains( '<amp-audio', $playlist );
 		$this->assertContains( $this->file_1, $playlist );