Validação não funciona

Boa noite, estou tentando fazer a validação dos campos do formulário, mas a validação não está acontecendo. DTO

import javax.validation.constraints.NotEmpty;
import javax.validation.constraints.NotNull;

import br.com.alura.forum.model.Curso;
import br.com.alura.forum.model.Topico;
import br.com.alura.forum.repository.CursoRepository;

public class TopicoForm {

    @NotNull @NotEmpty
    private String titulo;

    @NotNull @NotEmpty
    private String mensagem;

    @NotNull @NotEmpty
    private String nomeCurso;
...

Meu Controller

import java.net.URI;
import java.util.List;

import javax.validation.Valid;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.util.UriComponentsBuilder;

import br.com.alura.forum.controller.dto.TopicoDto;
import br.com.alura.forum.controller.dto.TopicoForm;
import br.com.alura.forum.model.Topico;
import br.com.alura.forum.repository.CursoRepository;
import br.com.alura.forum.repository.TopicoRepository;

@RestController
@RequestMapping("/topicos")
public class TopicosController{
...
    @PostMapping
    public ResponseEntity<TopicoDto> cadastrar(@RequestBody @Valid TopicoForm form, UriComponentsBuilder uriBuilder) {
        Topico topico = form.converter(cursoRepository);
        topicoRepository.save(topico);

        URI uri = uriBuilder.path("/topicos/{id}").buildAndExpand(topico.getId()).toUri();
        return ResponseEntity.created(uri).body(new TopicoDto(topico));
    }
...

meu pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.br.alura</groupId>
    <artifactId>forum</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>forum</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>


        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>