[infrastructure] add external null-annotations (#8848)

Signed-off-by: Jan N. Klug <jan.n.klug@rub.de>

bundles/org.openhab.binding.radiothermostat/src/main/java/org/openhab/binding/radiothermostat/internal/communication/RadioThermostatConnector.java

@@ -152,6 +152,10 @@ public class RadioThermostatConnector {
      * @return a valid URL for the thermostat's JSON interface
      */
     private String buildRequestURL(String resource) {
+        String hostName = this.hostName;
+        if (hostName == null) {
+            throw new IllegalStateException("hostname must not be null");
+        }
         String urlStr = URL.replace("%hostName%", hostName);
         urlStr = urlStr.replace("%resource%", resource);