Provide error message, catch warning, when Co-Authors Plus is active …

…but a post has no coauthors.

To reproduce this issue, see Automattic/Co-Authors-Plus#637

Part of #1492

inc/byline_class.php

@@ -262,10 +262,23 @@ function generate_byline() {
 			$authors = implode( ', ', array_slice( $out, 0, -1 ) );
 			$authors .= ' <span class="and">' . __( 'and', 'largo' ) . '</span> ' . $out[$key];
 		} else {
-			$authors = $out[0];
+			if ( isset( $out[0] ) ) {
+				$authors = $out[0];
+			} else {
+				$cap_error_message = sprintf(
+					esc_html__( 'post %1$s should have at least one co-author, but has none!', 'largo' ),
+					$this->post_id
+				);
+				if ( WP_DEBUG || LARGO_DEBUG ) {
+					error_log(var_export( $cap_error_message, true));
+				}
+				$authors = sprintf(
+					'<!-- %1$s -->',
+					$cap_error_message
+				);
+			}
 		}
 
-
 		// Now assemble the One True Byline
 		ob_start();
 		echo '<span class="by-author"><span class="by">' . __( 'By', 'largo' ) . '</span> <span class="author vcard" itemprop="author">' . $authors . '</span></span>';