Fix `UUIDConverter` nullability according to its superclass changes in Spring Framework

[artembilan]

The error is like this:

> Task :spring-integration-core:compileJava
\spring-integration\spring-integration-core\src\main\java\org\springframework\integration\util\UUIDConverter.java:52: error: [NullAway] method returns @Nullable, but superclass method org.springframework.core.convert.converter.Converter.convert(S) returns @NonNull
        public @Nullable UUID convert(@Nullable Object source) {
                              ^
    (see http://t.uber.com/nullaway )
1 error

[artembilan]

Related to: spring-projects/spring-framework#35947