Define default value for 'thumbnailError'

## Issue
"thumbnail is invalid" not reset with new thumbnail upload #6044
https://github.com/lbryio/lbry-desktop/issues/6044

## Change
Defining a default value will cover both CLEAR_PUBLISH and DO_PREPARE_EDIT

dist/bundle.es.js

@@ -6056,6 +6056,7 @@ const defaultState$4 = {
   thumbnail_url: '',
   thumbnailPath: '',
   uploadThumbnailStatus: API_DOWN,
+  thumbnailError: undefined,
   description: '',
   language: '',
   releaseTime: undefined,

src/redux/reducers/publish.js

@@ -22,6 +22,7 @@ type PublishState = {
   thumbnail_url: string,
   thumbnailPath: string,
   uploadThumbnailStatus: string,
+  thumbnailError: ?boolean,
   description: string,
   language: string,
   releaseTime: ?string,
@@ -55,6 +56,7 @@ const defaultState: PublishState = {
   thumbnail_url: '',
   thumbnailPath: '',
   uploadThumbnailStatus: THUMBNAIL_STATUSES.API_DOWN,
+  thumbnailError: undefined,
   description: '',
   language: '',
   releaseTime: undefined,