Undertow: ignore missing servlet class in web.xml

Avoids writing a null bean name to the bean archive, which causes a
later build failure.

Fixes quarkusio#43825

extensions/undertow/deployment/src/main/java/io/quarkus/undertow/deployment/WebXmlParsingBuildStep.java

@@ -79,7 +79,10 @@ WebMetadataBuildItem createWebMetadata(ApplicationArchivesBuildItem applicationA
                     }
                     if (metadata.getServlets() != null) {
                         for (ServletMetaData i : metadata.getServlets()) {
-                            additionalBeans.add(i.getServletClass());
+                            String servletClass = i.getServletClass();
+                            if (servletClass != null) {
+                                additionalBeans.add(servletClass);
+                            }
                         }
                     }
                     if (metadata.getFilters() != null) {