please see 1st comment below

DemoApp.java

package demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;

@Validated
@Controller
@SpringBootApplication
public class DemoApp {

    public static void main(String[] args) {
        SpringApplication.run(DemoApp.class, args);
    }

    @RequestMapping(
            method = RequestMethod.GET,
            value = "/todos/1/",
            produces = {"text/plain"}
    )
    public ResponseEntity<String> exampleOperation(
            @RequestParam(value = "par1", required = false, defaultValue = "true") Boolean par1,
            @RequestParam(value = "par2", required = false, defaultValue = "1.23") Float par2
    ) {
        return new ResponseEntity<>(par1 + " " + par2, HttpStatus.OK);
    }
}

pom.xml

<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 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-spring</artifactId>
    <name>openapi-spring</name>
    <version>1.0.0</version>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.1.4</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>
</project>

Purpose

Demo Spring Boot server which to show that even with defaultValue, parameters can be null.

How to run

1. move the java file to src/main/java/demo/DemoApp.java
2. run mvn compile exec:java '-Dexec.mainClass=demo.DemoApp'

Results

request url	response
http://localhost:8080/todos/1/?par1=true&par2=3&x=y	true 3.0
http://localhost:8080/todos/1/?par1=%20&par2=%20&x=y	null null

When using the latest Spring 6.1.0-M5, the 2nd request results in an HTTP status code 500.