####Problem
In {https://phabricator.wikimedia.org/T392898}, we defined that the fields Start date and End date should both be required, and should be validated as such on submit when configuring experiments and instruments. However, users need to be able to leave the end date empty when defining data collection instruments. This means that setting an End date should not be required for instruments, and users should be allowed to leave said field empty.
####Solution
We should make the End date field in the instrument configuration form optional.
####Acceptance criteria
[] Users can leave the End date field empty when configuring instruments
#### Technical notes
Just a few considerations we should keep in mind to work on this:
- End date (`utc_end_dt` column) is currently mandatory in the database (defined as `NOT NULL`)
- Once we fix the above, by removing the mentioned constraint, we will find that current frontend/backend is storing `utc_end_dt` as empty string (`''`) in the database and not as a `NULL` value, which causes error there because the empty string is not an allowed value for a `DATETIME` column. We should change something to set `utc_end_dt` as `NULL` when the user doesn't fill the corresponding field in the form