Import bookmarks - missing colour for highlights - "sanitize"/protect

[waluwaz]

Describe the bug
When Raindrop imports bookmarks that have highlights with no colour, Raindrop seems to create the highlight with no colour in storage. Fair enough.
On the screenshot below, the first highlight was imported. See how the cell for "colour" has no text. The other highlights/rows were created in Raindrop and do have an explicit colour appearing in the Raindrop export. (NB: Obviously the background colour in the spreadsheet was added by myself for illustration purposes.)

https://app.raindrop.io/my/0/helena/item/903788428/edit

But it seems Raindrop prefers when all highlights have a colour.
See #213

To Reproduce
Steps to reproduce the behavior:

1. Import a file where highlights are defined but don't have an explicit colour. (This seems typical of Diigo exports for instance).
2. See steps in Firefox extension - vertical scrollbar - missing markers for highlights #213

Suggestion
If Raindrop prefers that all highlights have an explicit colour, maybe define the colour at import time.
Or alternatively, ensure all features are robust enough to cope with missing/undefined colour.

Desktop (please complete the following information):
Not relevant. A Raindrop back-end feature.

Additional context
Thank you for that great tool.