column_name added as "null" when test modified

[JanPeterDatakind]

Describe the bug
If a test which takes parameters is edited through the UI and the column is left blank (intentionally), the resulting entry in the dot.configured_tests tables has its column_name set to "null" which breaks the execution of the dot run/ creation of test results

To Reproduce
Steps to reproduce the behavior on the demo ScanProject1 project:

1. Go to test screen
2. Edit test "duplicate flight record" and leave column name parameter empty
3. Go to DBeaver/ DB management software of your choice and inspect the dot.configured_tests table
4. See value "null" in the column_name column

Expected behavior
A clear and concise description of what you expected to happen.
Instead of adding "null", this field should be left empty in the DB if it has been left empty in the UI

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.