UPDATE: Maybe it's not possible:
"... Where @RequestParam annotations can be used to make a parameter required or with default value, in command objects this is done with bean validation constraints like @NotNull and plain Java/Kotlin. To activate bean validation, the method argument has to be annotated with @Valid.
When the bean validation fails, a BindException or WebExchangeBindException in the reactive stack is thrown. Both exceptions implement BindingResult, which provides nested errors for each invalid field value. Above controller method would result in error messages like... "
See "Request Validation" in https://medium.com/sprang/validation-and-exception-handling-with-spring-ba44b3ee0723
UPDATE: Maybe it's not possible: "... Where @RequestParam annotations can be used to make a parameter required or with default value, in command objects this is done with bean validation constraints like @NotNull and plain Java/Kotlin. To activate bean validation, the method argument has to be annotated with @Valid. When the bean validation fails, a BindException or WebExchangeBindException in the reactive stack is thrown. Both exceptions implement BindingResult, which provides nested errors for each invalid field value. Above controller method would result in error messages like... "
UPDATE: try to use @NotNull
UPDATE: How can be the DTO null?