docs: Added expressions for validating if a field is empty or not (#4353

)

* Updated docs to include info regarding on validating is field is empty or not

* Rewrote explanation a bit to include reference to required

docs/molgenis/use_schema.md

@@ -282,6 +282,14 @@ expression itself is shown. Otherwise, the return value of the expression will b
 | `/^([a-z]+)$/.test(name)`                                                  | Application of validation rule failed: /^([a-z]+)$/.test(name)                    |
 | `if(!/^([a-z]+)$/.test(name))'name should contain only lowercase letters'` | Application of validation rule failed: name should contain only lowercase letters |
 
+Special attention needs to be paid when validating if a field is empty or not (as filled in fields that get emptied are different from never filled in fields).
+While [required](#required) should be used to ensure a field itself is filled, when creating expressions (that include other fields), use the following:
+
+| validation               | functioning                       |
+|--------------------------|-----------------------------------|
+| `columnName?.length > 0` | Field 'columnName' must be filled |
+| `!(columnName?.length)`  | Field 'columnName' must be empty  |
+
 Visible expressions must return a value that is not false or undefined, otherwise the column stays hidden in the user interface. In the event that javascript
 throws an exception, this is shown in user interface/error message. For example: