Add default clade color

[joverlee521]

Description of proposed changes

Remove the assertion that has been blocking the pipeline due to lack of clade definition for 21L/BA.2.86.¹

Adds a default dark grey color for clades that lack clade definition so that we can see them on the forecasts site. In the future, we may want to add a custom Slack notification fo these clades so that we get a louder internal notification.

¹ https://bedfordlab.slack.com/archives/C036L7V615F/p1701195514961829

Related issue(s)

Previous conversation for context

Checklist

• [x] Checks pass
• [x] Test run

[joverlee521]

The 21L clade definitely sticks out in the viz from the trial run: