Updated: Enhance null safety and set default URL in EditMonitor.vue

- Added optional chaining to safely access `this.monitor.hostname`.
- Introduced default URL (`https://`) assignment to `this.monitor.url`.
- Ensured `hostname` is trimmed after the URL is set.

Fixes [louislam#5466](louislam#5466)

modified: src/pages/EditMonitor.vue

src/pages/EditMonitor.vue

@@ -1719,7 +1719,8 @@ message HealthCheckResponse {
                 this.monitor.headers = JSON.stringify(JSON.parse(this.monitor.headers), null, 4);
             }
 
-            if (this.monitor.hostname) {
+            if (this.monitor?.hostname) {
+                this.monitor.url = "https://";
                 this.monitor.hostname = this.monitor.hostname.trim();
             }