[MM-432] Fix issue of todo getting assigned to last assigned user

[ayusht2810]

Summary

• Fixed issue of todo getting assigned to last assigned user
• Can close unifinshed PR fix(issue#215) : clear state assignee before close #226

Ticket Link

• Fixes Keeping the assign field empty auto-assigns the ToDo. #215

What to test?

• Assignee field is cleared and the old user is not assigned to the new todo after creating a todo

Checklist

• Completed dev testing
• make test Ran test cases and ensured they are passing
• make check-style Ran style check and ensured both webapp and server pass the checks

[ayusht2810]

@mickmister Fixed the review fixes. Please re-review.

[arush-vashishtha]

The above PR has been tested for the following scenarios:

• Tested by keeping the Assignee field clear for todo
• Tested that the old user is not assigned to the new todo after creating a todo

The PR was working fine for both the conditions, LGTM. Approved