Handle null additional properties more gracefully

Closes gh-30654

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/buildinfo/BuildInfoTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -33,6 +33,7 @@ import org.springframework.boot.gradle.junit.GradleProjectBuilder;
 import org.springframework.boot.testsupport.classpath.ClassPathExclusions;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * Tests for {@link BuildInfo}.
@@ -129,6 +130,14 @@ class BuildInfoTests {
 		assertThat(buildInfoProperties(task)).containsEntry("build.b", "bravo");
 	}
 
+	@Test
+	void nullAdditionalPropertyProducesInformativeFailure() {
+		BuildInfo task = createTask(createProject("test"));
+		task.getProperties().getAdditional().put("a", null);
+		assertThatThrownBy(() -> buildInfoProperties(task))
+				.hasMessage("Additional property 'a' is illegal as its value is null");
+	}
+
 	private Project createProject(String projectName) {
 		File projectDir = new File(this.temp, projectName);
 		Project project = GradleProjectBuilder.builder().withProjectDir(projectDir).withName(projectName).build();