Webpublisher: added check for empty context

[kalisp]

Brief description

Safer check for presence of "context" data in batch manifest.

Description

Currently is somehow possible to upload batch without context data. This results in uncaught fail and batch keeps in "in_progress".

Testing notes:

1. provide batch with manifest with empty context ({})
2. run WP

[ynbot]

Task linked: OP-3722 WP FE shouldnt be able to send batch without context