Previously when using the multiline option for text edit widgets
the entire contents of the field would be discarded if the entered
value exceeded the maximum length for a string field.
Now the entered string is truncated to the maximum field length.
* Add null handling to value map edit widget (fixes#15215)
* Return QVariant type
* Use hardcoded value for 'null' representation
* Detect "null" value when loading value map from csv; use null QString constructor
* Use configured "null" representation for display in value map
* Use single definition for value map null representation guid
* Added unit test for value map widget and fixed value displaying bug
