fix(monitor): handle empty headers

server/model/monitor.js

@@ -148,10 +148,10 @@ class Monitor extends BeanModel {
                         headers: {
                             "Accept": "*/*",
                             "User-Agent": "Uptime-Kuma/" + version,
-                            ...JSON.parse(this.headers),
+                            ...(this.headers ? JSON.parse(this.headers) : {}),
                         },
                         httpsAgent: new https.Agent({
-                            maxCachedSessions: 0,      // Use Custom agent to disable session reuse (https://github.com/nodejs/node/issues/3940)
+                            maxCachedSessions: 0,      // Use "{}"om agent to disable session reuse (https://github.com/nodejs/node/issues/3940)
                             rejectUnauthorized: ! this.getIgnoreTls(),
                         }),
                         maxRedirects: this.maxredirects,