Display name: return "" if first or last name is not set in LDAP/AD

Change the return API to avoid displaying "null" as name in every client
Usefull when first and/or last name is left blank during LDAP/AD creation

GitLab: #145
Change-Id: I48270ad514d6ffaa33c58678354b2fc146d5d18f

jams-server/src/main/java/net/jami/jams/server/core/workflows/RegisterDeviceFlow.java

@@ -85,7 +85,11 @@ public class RegisterDeviceFlow {
                             user.getEthAddress());
             response.setDeviceReceipt(devReceipt[0]);
             response.setReceiptSignature(devReceipt[1]);
-            response.setDisplayName(userProfile.getFirstName() + " " + userProfile.getLastName());
+            String firstDisplayName =
+                    userProfile.getFirstName() != null ? userProfile.getFirstName() : "";
+            String lastDisplayName =
+                    userProfile.getLastName() != null ? userProfile.getLastName() : "";
+            response.setDisplayName(firstDisplayName + " " + lastDisplayName);
 
             response.setNameServer(nameServer.getURI());
             if (userProfile.getProfilePicture() != null)