table: fix import/export button visibility

Author: lyubov-voloshko

frontend/src/app/components/dashboard/db-table/db-table.component.html

@@ -110,7 +110,7 @@ <h2 class="mat-h2 table-name">{{ displayName }}</h2>
                     </mat-menu>
                 </div>
                 <button mat-icon-button type="button"
-                    *ngIf="accessLevel === 'edit'"
+                    *ngIf="accessLevel === 'edit' && tableData && (tableData.isImportAllowed || tableData.isExportAllowed)"
                     matTooltip="Emport / Export"
                     [matMenuTriggerFor]="transferDataMenu"
                     angulartics2On="click"