Add check for empty comments and notes

[Ashesh3]

Builds on coronasafe/care_fe#2525

Adds a check for empty comments and notes from the backend and rejects with proper validation message when needed.

Also some minor formatting change performed automatically by pre-commit-hooks

[sainak]

We can do this without regex

[sainak]

LGTM

[vigneshhari]

I would create a new serializer field that has the empty check validator built-in and use that everywhere instead of re-writing the logic multiple times. Create a PR with this or create an issue to create such a field.

[vigneshhari]

Changes look good, Suggestions for the next iteration provided above, Untested.