Polish "Handle null Principal in AuditEvent"

Closes gh-11320

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/audit/AuditEvent.java

@@ -41,7 +41,6 @@ import org.springframework.util.Assert;
  * (wrappers for AuditEvent).
  *
  * @author Dave Syer
- * @author Nakul Mishra
  * @see AuditEventRepository
  */
 @JsonInclude(Include.NON_EMPTY)
@@ -117,7 +116,8 @@ public class AuditEvent implements Serializable {
 	}
 
 	/**
-	 * Returns the user principal responsible for the event.
+	 * Returns the user principal responsible for the event or an empty String if the
+	 * principal is not available.
 	 * @return the principal
 	 */
 	public String getPrincipal() {

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/audit/AuditEventTests.java

@@ -32,7 +32,6 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Dave Syer
  * @author Vedran Pavic
- * @author Nakul Mishra
  */
 public class AuditEventTests {
 
@@ -56,6 +55,13 @@ public class AuditEventTests {
 		assertThat(event.getData().get("c")).isEqualTo("d");
 	}
 
+	@Test
+	public void nullPrincipalIsMappedToEmptyString() {
+		AuditEvent auditEvent = new AuditEvent(null, "UNKNOWN",
+				Collections.singletonMap("a", (Object) "b"));
+		assertThat(auditEvent.getPrincipal()).isEmpty();
+	}
+
 	@Test
 	public void nullTimestamp() throws Exception {
 		this.thrown.expect(IllegalArgumentException.class);
@@ -64,12 +70,6 @@ public class AuditEventTests {
 				Collections.singletonMap("a", (Object) "b"));
 	}
 
-	@Test
-	public void nullPrincipal() {
-		AuditEvent auditEvent = new AuditEvent(null, "UNKNOWN", Collections.singletonMap("a", (Object) "b"));
-		assertThat(auditEvent.getPrincipal()).isEmpty();
-	}
-
 	@Test
 	public void nullType() throws Exception {
 		this.thrown.expect(IllegalArgumentException.class);