Fix attendee naming handling

optString returns "null" for null values, which makes "null" visible in
the UI.

Author: antweb

pretixscan/app/src/main/java/eu/pretix/pretixscan/droid/ui/MainActivity.kt

@@ -1015,7 +1015,12 @@ class MainActivity : AppCompatActivity(), ReloadableActivity, ScannerView.Result
         } else {
             values
         }
-        val attendeeName = if (conf.hideNames) "" else res.position?.optString("attendee_name")
+        val attendeeName = if (conf.hideNames || res.position?.isNull("attendee_name") != false) {
+            ""
+        } else {
+            res.position?.optString("attendee_name")
+        }
+
         var attendeeDOB: String? = null
         if (!conf.hideNames) {
             val qlen = res.position?.getJSONArray("answers")?.length() ?: 0