[Frontend] Consistent null values

[ds-crehm]

As user,
I want null values to be shown in a user-friendly way,
so that I understand the values I'm seeing and can work with them.

Hints / Details

Frontend implementation of #578

Acceptance Criteria

• Null values are shown as a dash "-" in the frontend.
• When sorting columns, null values are treated the default way of the database. (Shown at the end for ASC - shown at the beginning for DESC)
• Null values can be filtered for. They are shown as "-" when filtering for them. This option is always shown at the top of the filter/search box regardless of what the user is searching for.
• Leading and trailing whitespace characters don't count as mandatory values during user input.

Out of Scope

Any further string validation is out of scope. For example values like "-", "--", ".", "..." will not be changed and instead written into the database and shown in the frontend unchanged.