Fix favIcons of opened ZIM files (not added via catalog download)

[BPerlakiH]

Fixes: #789

[kelson42]

@BPerlakiH CI does not pass

[kelson42]

@BPerlakiH What isnthenlogic of usage between the remote URL and the local illustration?

[BPerlakiH]

@kelson42 I have updated the code, it seems that empty string to empty data was not handled properly.

What is the logic of usage between the remote URL and the local illustration?

We can create these instances / entities 2 ways:

• via catalog download, where it will take the favicon URL from the catalog, and as it is rendered (when browsing the catalog) it will update the entity with the favicon image data downloaded from that URL (network request).
  Once we have this ZIM file from the catalog the favicon URL and the image data is stored in the local DB and we are good.
• via adding the ZIM file from local disk. In this case it will read the image data from the ZIM file itself, and "upsert" that into the DB, and that will be used to render the favicon.

[kelson42]

AFAP usenonline catalogue metadata (even in local library) and otherwise the local (from ZIM file).

[BPerlakiH]

If the url is set, that will be displayed, if not it will fallback to the image data, so that covers the scenario you mentioned.