Store NULL for empty descriptions and comments in the groups database tables

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
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• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

Empty comments and descriptions should be stored as NULL in the database.

How does this PR accomplish the above?:

Check if the input fields are empty and, if so, store NULL. Storing NULL through bindValue() with SQLITE3_TEXT is explicitly supported and what we're doing already when editing. The code has just been missing for newly added entries.

You can check whether adding a new group with NULL descriptions worked using, e.g.,

sqlite3 /etc/pihole/gravity.db "SELECT * FROM \"group\" WHERE description IS NULL;"

What documentation changes (if any) are needed to support this PR?:

None

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/add-group-overview-to-group-management/31323/16

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-5-1-released/35577/1