wip: add png image bytes for mock

Author: sanak

example/lib/mock_project_db.dart

@@ -82,6 +82,15 @@ class MockProjectDb implements ProjectDb {
       ..text = "log_20221202_000000"
   ];
 
+  // 1x1 transparent pixel
+  // https://github.com/flutter/flutter/blob/master/packages/flutter_test/test/goldens_test.dart#L14-L20
+  Uint8List pngBytes = Uint8List.fromList([
+    137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82, 0, 0, 0,
+    1, 0, 0, 0, 1, 8, 6, 0, 0, 0, 31, 21, 196, 137, 0, 0, 0, 11, 73, 68, 65, 84,
+    120, 1, 99, 97, 0, 2, 0, 0, 25, 0, 5, 144, 240, 54, 245, 0, 0, 0, 0, 73, 69,
+    78, 68, 174, 66, 96, 130, // avoid flutter format
+  ]);
+
   @override
   String getPath() {
     return "mock/path";
@@ -145,22 +154,22 @@ class MockProjectDb implements ProjectDb {
 
   @override
   Image? getThumbnail(int imageDataId) {
-    return null;
+    return Image.memory(pngBytes);
   }
 
   @override
   Uint8List? getThumbnailBytes(int imageDataId) {
-    return null;
+    return pngBytes;
   }
 
   @override
   Image? getImage(int imageDataId) {
-    return null;
+    return Image.memory(pngBytes);
   }
 
   @override
   Uint8List? getImageDataBytes(int imageDataId) {
-    return null;
+    return pngBytes;
   }
 
   @override