Prevent NPE when using pathExtension predicate

This commit ensures pathExtension predicate is skipped when the value
is null, in order to provide a more predictable behavior, and allow
a better compatibility with collections not supporting null elements
like the ones created by List#of.

Closes gh-32404

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RequestPredicates.java

@@ -843,7 +843,7 @@ public abstract class RequestPredicates {
 		@Override
 		public boolean test(ServerRequest request) {
 			String pathExtension = UriUtils.extractFileExtension(request.path());
-			return this.extensionPredicate.test(pathExtension);
+			return (pathExtension != null && this.extensionPredicate.test(pathExtension));
 		}
 
 		@Override