gui: Disable Restore Versions filters when no versioned files exist (fixes #5408) (#8539)

Currently, the name and date filters in the Restore Versions modal are
always enabled, even if there are no versioned files present. With this
change, they are enabled only when there are no errors and versioned
files actually exist.

In addition to disabling the filters, also completely skip date picker
generation, as it serves no function while still displaying a non-sense
date range, which starts today and ends in the past.

Signed-off-by: Tomasz Wilczyński <twilczynski@naver.com>

gui/default/syncthing/core/syncthingController.js

@@ -2686,6 +2686,8 @@ angular.module('syncthing.core')
                             if (closed) {
                                 resetRestoreVersions();
                                 return;
+                            } else if ($scope.sizeOf($scope.restoreVersions.versions) === '0') {
+                                return;
                             }
 
                             $scope.restoreVersions.tree = $("#restoreTree").fancytree({
@@ -2702,7 +2704,7 @@ angular.module('syncthing.core')
                                     indentation: 24,
                                 },
                                 strings: {
-                                    loading: $translate.instant("Loading..."),
+                                    loading: $translate.instant("Loading data..."),
                                     loadError: $translate.instant("Failed to load file versions."),
                                     noData: $translate.instant("There are no file versions to restore.")
                                 },