Fix error when changing dates in the position history

[gjvoosten]

Closes AB#722

User changes

• None.

Superuser changes

• Changing start or end dates when manually editing position history works correctly again.

Admin changes

• None.

System admin changes

• [ ] anet.yml or anet-dictionary.yml needs change
• [ ] db needs migration
• [ ] documentation has changed
• [ ] graphql schema has changed

Checklist

• [x] Described the user behavior in PR body
• [x] Referenced/updated all related issues
• [x] commits follow a repo#issue: Title title format and these 7 rules
• [x] commits have a clean history, otherwise PR may be squash-merged
• [ ] Added and/or updated unit tests
• [x] Added and/or updated e2e tests
• [ ] Added and/or updated data migrations
• [ ] Updated documentation
• [x] Resolved all build errors and warnings
• [ ] Opened debt issues for anything not resolved here