Allow user to remove next increase ask date

[dr-bizz]

Description

In this PR, I allow the user to remove the Next Increase Ask date. A Help Scout ticket brought this to my attention.
I also noticed this was happening to the Start Date field, which I fixed.

When adding tests, I cleaned up the page.

To Test:

1. On a contact, click on the Donations tab, scroll down and then click on the Partnership info tab.
2. Edit partnership to show modal.
3. Set a Next Increase Ask date if there isn't one.
4. On the Next Increase Ask field, click the calendar icon, and then click CLEAR.
5. Click Save

On production, doing the above will not remove the Next Increase Ask date.
On the preview link, the Next Increase Ask date will be removed.

HelpScout Ticket
Jira Task

Checklist:

• I have given my PR a title with the format "MPDX-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels. (Add the label "On Staging" to get the branch automatically merged into staging.)
• I have requested a review from another person on the project

[github-actions]

Preview branch generated at https://allow-user-to-remove-next-increase-ask-date.d3dytjb8adxkk5.amplifyapp.com

[github-actions]

Bundle sizes [mpdx-react]

Compared against dbfb393

No significant changes found

[canac]

Tested, and it works! Thanks for the fix.