docs: Added expressions for validating if a field is empty or not

[svandenhoek]

What are the main changes you did:

Added docs describing the following scenarios:

1. Validating that a field must be filled

Schermopname.2024-10-14.om.12.19.42.mov

2. Validating that a field must be empty

Schermopname.2024-10-14.om.12.20.31.mov

how to test:

• Create schema/table where c2 has the validation as mentioned in the docs referring to c1 (see also videos where the error message also shows the used expression).

todo:

• updated docs in case of new feature
• added/updated tests
• added/updated testplan to include a test for this fix, including ref to bug using # notation

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud