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

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

bundles/org.openhab.binding.fmiweather/src/main/java/org/openhab/binding/fmiweather/internal/client/Client.java

@@ -83,7 +83,7 @@ public class Client {
     }
     private static final NamespaceContext NAMESPACE_CONTEXT = new NamespaceContext() {
         @Override
-        public String getNamespaceURI(@Nullable String prefix) {
+        public @Nullable String getNamespaceURI(@Nullable String prefix) {
             return NAMESPACES.get(prefix);
         }