MLE-25666 Fixing small logging issue

"null" was being logged for security API calls when doing cloud auth. No longer.

Author: rjrudin

ml-app-deployer/src/main/java/com/marklogic/mgmt/ManageClient.java

@@ -280,7 +280,7 @@ protected String determineUsernameForSecurityUserRequest() {
 				username = manageConfig.getUsername();
 			}
 		}
-		return username;
+		return username == null ? "" : username;
 	}
 
 	private void initializeSecurityUserRestTemplate() {

ml-app-deployer/src/test/java/com/marklogic/mgmt/ManageClientTest.java

@@ -24,6 +24,14 @@ void determineUsernameForSecurityUserRequest() {
 		assertEquals("someone", client.determineUsernameForSecurityUserRequest());
 	}
 
+	@Test
+	void nullUsername() {
+		// Will be the case when doing cloud auth.
+		ManageConfig config = new ManageConfig("localhost", 8002, null, null);
+		ManageClient client = new ManageClient(config);
+		assertEquals("", client.determineUsernameForSecurityUserRequest());
+	}
+
 	@Test
 	void nullManageConfig() {
 		ManageClient client = new ManageClient((ManageConfig) null);