Show username if full_name or last_name is empty.

important for create-example_data

client/src/app/shared/models/users/user.ts

@@ -87,11 +87,11 @@ export class User extends BaseModel<User> implements Searchable {
             shortName = `${title} ${shortName}`;
         }
 
-        return shortName;
+        return shortName || this.username;
     }
 
     public getTitle(): string {
-        return this.full_name;
+        return this.full_name || this.username;
     }
 
     public getListViewTitle(): string {

client/src/app/site/agenda/models/view-speaker.ts

@@ -43,7 +43,7 @@ export class ViewSpeaker extends BaseViewModel implements Selectable {
     }
 
     public get name(): string {
-        return this.user.full_name;
+        return this.user.full_name || this.user.username;
     }
 
     public constructor(speaker?: Speaker, user?: User) {

client/src/app/site/users/models/view-user.ts

@@ -36,11 +36,11 @@ export class ViewUser extends BaseProjectableModel {
     }
 
     public get full_name(): string {
-        return this.user ? this.user.full_name : null;
+        return this.user ? this.user.full_name || this.username : null;
     }
 
     public get short_name(): string {
-        return this.user ? this.user.short_name : null;
+        return this.user ? this.user.short_name || this.username : null;
     }
 
     public get email(): string {