Ensure that a PHP Compatibility report exists from WP Tide before att…

…empting to use it, to avoid an undefined property warning.

Fixes #466

HealthCheck/Tools/class-health-check-plugin-compatibility.php

@@ -140,7 +140,7 @@ function get_supported_php( $slug, $version ) {
 
 			$json = json_decode( $tide_response );
 
-			if ( empty( $json ) ) {
+			if ( empty( $json ) || ! isset( $json->reports->phpcs_phpcompatibilitywp->report->compatible ) ) {
 				$tide_versions = array();
 			} else {
 				$tide_versions = $json->reports->phpcs_phpcompatibilitywp->report->compatible;