Nexus: Adding a completion to a Wellspec file with no Time card present fails to create a new one

If add_completion is called on a wellspec file that doesn't contain any time cards, the new completion will be added to the same table as any existing completions for that well.

Expected behaviour: A new time card is added, as well as a new completion table, which the new completion is added to.

I have added a test to the branch `fix_add_completion_no_time_card` that fails due to this to help explain the issue and assist with fixing it. Please feel free to add other test cases in, however.

Start with the add_completion entry point and inspect the failing test on the fix_add_completion_no_time_card branch. Reproduce the case using a Wellspec file with no time cards, then verify that a new time card and completion table are created and the completion is added to that table.

Written by the indexing model from the issue text.