Send 400 for PathVariable that is null after conversion

This implies a value was actually sent, but is not something
that can be converted to the expected type.

Closes gh-31382

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/bind/MissingPathVariableException.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -88,7 +88,7 @@ public final MethodParameter getParameter() {
 
 	@Override
 	public HttpStatusCode getStatusCode() {
-		return HttpStatus.INTERNAL_SERVER_ERROR;
+		return (isMissingAfterConversion() ? HttpStatus.BAD_REQUEST : HttpStatus.INTERNAL_SERVER_ERROR);
 	}
 
 }