Fix: JSON parsing in backend_srv (#45598)

* fix(plugins/state): console log errors during install / uninstall

* fix(backend_srv): catch JSON parse errors

Sometimes it can happen that a backend API responses says that it's
returning with a JSON content-type, however it actually returns an invalid
JSON (e.g. an empty body) - in which case the backendSrv() request errors out.

public/app/core/utils/fetch.ts

@@ -105,7 +105,12 @@ export async function parseResponseBody<T>(
         return response.blob() as any;
 
       case 'json':
-        return response.json();
+        try {
+          return await response.json();
+        } catch (err) {
+          console.warn(`${response.url} returned an invalid JSON -`, err);
+          return {} as unknown as T;
+        }
 
       case 'text':
         return response.text() as any;