Index: src/wp-admin/includes/class-wp-users-list-table.php
===================================================================
--- src/wp-admin/includes/class-wp-users-list-table.php	(revision 40566)
+++ src/wp-admin/includes/class-wp-users-list-table.php	(working copy)
@@ -470,7 +470,11 @@
 						$r .= "$avatar $edit";
 						break;
 					case 'name':
-						$r .= "$user_object->first_name $user_object->last_name";
+					  if ( ! empty( $user_object->first_name ) || ! empty( $user_object->last_name ) ) {
+							$r .= "$user_object->first_name $user_object->last_name";
+						} else {
+							$r .= _x('Unknown');
+						}
 						break;
 					case 'email':
 						$r .= "<a href='" . esc_url( "mailto:$email" ) . "'>$email</a>";