Return an empty string as default if no logging is enabled, to not ca…

…use a deprecation warning from `esc_textarea()`.

Fixes #473

HealthCheck/Tools/class-health-check-debug-log-viewer.php

@@ -11,7 +11,7 @@ public function __construct() {
 
 	private function read_debug_log() {
 		if ( ! defined( 'WP_DEBUG_LOG' ) || false === WP_DEBUG_LOG ) {
-			return null;
+			return '';
 		}
 
 		$logfile = WP_DEBUG_LOG;
@@ -25,7 +25,7 @@ private function read_debug_log() {
 		}
 
 		if ( ! file_exists( $logfile ) ) {
-			return null;
+			return '';
 		}
 
 		$debug_log = @file_get_contents( $logfile );