Handle null Principal in AuditEvent

See gh-11320

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

@@ -41,6 +41,7 @@ import org.springframework.util.Assert;
  * (wrappers for AuditEvent).
  *
  * @author Dave Syer
+ * @author Nakul Mishra
  * @see AuditEventRepository
  */
 @JsonInclude(Include.NON_EMPTY)
@@ -85,10 +86,9 @@ public class AuditEvent implements Serializable {
 	public AuditEvent(Date timestamp, String principal, String type,
 			Map<String, Object> data) {
 		Assert.notNull(timestamp, "Timestamp must not be null");
-		Assert.notNull(principal, "Principal must not be null");
 		Assert.notNull(type, "Type must not be null");
 		this.timestamp = timestamp;
-		this.principal = principal;
+		this.principal = principal != null ? principal : "";
 		this.type = type;
 		this.data = Collections.unmodifiableMap(data);
 	}

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

@@ -32,6 +32,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Dave Syer
  * @author Vedran Pavic
+ * @author Nakul Mishra
  */
 public class AuditEventTests {
 
@@ -64,10 +65,9 @@ public class AuditEventTests {
 	}
 
 	@Test
-	public void nullPrincipal() throws Exception {
-		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("Principal must not be null");
-		new AuditEvent(null, "UNKNOWN", Collections.singletonMap("a", (Object) "b"));
+	public void nullPrincipal() {
+		AuditEvent auditEvent = new AuditEvent(null, "UNKNOWN", Collections.singletonMap("a", (Object) "b"));
+		assertThat(auditEvent.getPrincipal()).isEmpty();
 	}
 
 	@Test