web/admin: fix diff showing previous false as "-" (#13580)

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

web/src/components/ak-event-info.ts

@@ -271,7 +271,8 @@ export class EventInfo extends AKElement {
                         <tbody role="rowgroup">
                             ${Object.keys(diff).map((key) => {
                                 const value = diff[key];
-                                const previousCol = value.previous_value
+                                const previousCol =
+                                    value.previous_value !== null
                                         ? JSON.stringify(value.previous_value, null, 4)
                                         : msg("-");
                                 let newCol = html``;