🐞[BUG] Null value for end date shown as 1970-01-01

[ds-crehm]

Description

Happened on E2E:
Instead of being empty or showing '-' or 'null', empty values in the end date of contracts are shown as 1970-01-01 00:00, which might cause confusion for users.

Current behaviour

The end date for the following contracts is null:

In the frontend it is shown as '1970-01-01 00:00':

Expected behavior

I would expect them shown either empty, as 'null' or in the best case as '-' like described in this story: #759

Steps to reproduce the Bug

1. Log in as administrator
2. Go to contracts view