Fix exception when there are no more tuitions to pay

[LuisDuarte1]

Closes #817

Fixes bug when there are no more tuitions to pay and causes the worker to be rescheduled and be ran more often.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[LuisDuarte1]

Also, I don't like the logic of returning Sem data when there are no tuitions available, this makes the app more difficult to translate, should I handle it here?

[bdmendes]

@LuisDuarte1 absolutely: it also makes the value very prone to bugs like these. If possible, refactor the parser to return a DateTime.

[LuisDuarte1]

@LuisDuarte1 absolutely: it also makes the value very prone to bugs like these. If possible, refactor the parser to return a DateTime.

Done

[Sirze01]

Well done. Good catch on converting the due date to a DateTime in both places.