[miio] avoid NPE when rssi is missing (#10854)

Signed-off-by: Marcel Verpaalen <marcel@verpaalen.com>

bundles/org.openhab.binding.miio/src/main/java/org/openhab/binding/miio/internal/MiIoInfoApDTO.java

@@ -55,7 +55,7 @@ public class MiIoInfoApDTO {
     }
 
     public Long getRssi() {
-        if (rssi.isNumber()) {
+        if (rssi != null && rssi.isNumber()) {
             return rssi.getAsLong();
         }
         return null;