* FieldDisplay: added support for value mapping of no data state.

 Committer: Marcus Andersson <marcus.andersson@grafana.com>

* FieldDisplay: fixed issue when switching between modes and display numeric was null.

* ValueMapping: introduced a private function for checking null values.

* FieldDisplay: refactoring of test setup to reduce duplication.

* Docs: added info about new 'no data' value to text mapping.

* Docs: improved according to feedback. Reverted prettier formatting changes.

* FieldDisplay: removed unused import.