Fix minor bug with checkboxes in a table
What does this MR do and why?
In a table in the rich text editor, allow inserting a checkbox that doesn't have
any label by adding a   to it.
Screenshots or screen recordings
| Before | After | 
|---|---|
|  |  | 
How to set up and validate locally
- Enable feature flag :content_editor_on_issues.
- Write a new comment on an issue and switch to the rich text editor using the dropdown on the bottom right corner.
- Create a table from the table icon in the toolbar.
- Add a checkbox to a row using the task list icon in the toolbar.
- Save.
Note that previously, the task item would not render because it didn't have a label. Now it should render fine.
MR acceptance checklist
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I have evaluated the MR acceptance checklist for this MR. 
Edited  by Himanshu Kapoor