[SIEM] bug when you delete column on timeline

[XavierM]

Summary

When a user deletes a column on the timeline, this column became the sorting columns. This should not happen.

And this bug also brought up that it is possible for a user to sort column where all the value are null. and that's ok if user wants to do that. So we changed the type in graphql to allow it.

We might also want to think about this situation. If the column that we are going to delete is the sorting column/field in the timeline, do we want to pick a new sorting column/field in the timeline for less confusion? any thought? By the way, I do not mind to add it in this PR but just want to make sure that we are all on the same page.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• [ ] Unit or functional tests were updated or added to match the most common scenarios
• [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

• [ ] This was checked for breaking API changes and was labeled appropriately
• [ ] This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/secops

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[angorayc]

@XavierM, thank you for fixing this! Checked out the code and no issue found. For me I think it is fine to leave it like this without picking another sorting field, but would be helpful to me if there is a prompt to remind me before deleting the sorting field.