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

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

bundles/org.openhab.binding.exec/src/main/java/org/openhab/binding/exec/internal/handler/ExecHandler.java

@@ -422,6 +422,7 @@ public class ExecHandler extends BaseThingHandler {
     }
 
     public static String getOperatingSystemName() {
-        return System.getProperty("os.name");
+        String osname = System.getProperty("os.name");
+        return osname != null ? osname : "unknown";
     }
 }