upgrade NullSafe dependencies

pubspec.yaml

@@ -14,16 +14,16 @@ dependencies:
     sdk: flutter
   quill_delta: ^2.0.0
   quiver_hashcode: ^2.0.0
-  collection: ^1.14.13
-  tuple: ^1.0.3
-  url_launcher: ^5.7.10
-  flutter_colorpicker: ^0.3.5
-  image_picker: ^0.6.7+22
+  collection: ^1.15.0
+  tuple: ^2.0.0-nullsafety.0
+  url_launcher: ^6.0.0
+  flutter_colorpicker: ^0.4.0-nullsafety.0
+  image_picker: ^0.7.0
   photo_view: ^0.10.3
-  universal_html: ^1.2.1
-  file_picker: ^2.1.6
-  string_validator: ^0.1.4
-  flutter_keyboard_visibility: ^4.0.4
+  universal_html: ^1.2.4
+  file_picker: ^3.0.0-nullsafety.2
+  string_validator: ^0.2.0-nullsafety.0
+  flutter_keyboard_visibility: ^5.0.0-nullsafety.2
 
 
 dev_dependencies: