Linter: fix prefer_is_empty suggestions and change it to warning

analysis_options.yaml

@@ -31,6 +31,7 @@ analyzer:
     avoid_empty_else: error
     exhaustive_cases: error
     unused_import: warning
+    prefer_is_empty: warning
     use_rethrow_when_possible: info
 
     prefer_const_constructors: ignore # too many warnings

lib/ui/huge_listview/lazy_loading_gallery.dart

@@ -146,7 +146,7 @@ class _LazyLoadingGalleryState extends State<LazyLoadingGallery> {
 
   @override
   Widget build(BuildContext context) {
-    if (_files.length == 0) {
+    if (_files.isEmpty) {
       return Container();
     }
     return Padding(

lib/ui/shared_collections_gallery.dart

@@ -65,7 +65,7 @@ class _SharedCollectionGalleryState extends State<SharedCollectionGallery>
           final c =
               CollectionsService.instance.getCollectionByID(file.collectionID);
           if (c.owner.id == Configuration.instance.getUserID()) {
-            if (c.sharees.length > 0) {
+            if (c.sharees.isNotEmpty) {
               outgoing.add(
                 CollectionWithThumbnail(
                   c,
@@ -114,7 +114,7 @@ class _SharedCollectionGalleryState extends State<SharedCollectionGallery>
             Padding(padding: EdgeInsets.all(6)),
             SectionTitle("incoming"),
             Padding(padding: EdgeInsets.all(16)),
-            collections.incoming.length > 0
+            collections.incoming.isNotEmpty
                 ? GridView.builder(
                     shrinkWrap: true,
                     physics: NeverScrollableScrollPhysics(),
@@ -133,7 +133,7 @@ class _SharedCollectionGalleryState extends State<SharedCollectionGallery>
             Padding(padding: EdgeInsets.all(14)),
             SectionTitle("outgoing"),
             Padding(padding: EdgeInsets.all(16)),
-            collections.outgoing.length > 0
+            collections.outgoing.isNotEmpty
                 ? Padding(
                     padding: const EdgeInsets.fromLTRB(12, 0, 0, 0),
                     child: ListView.builder(