Value Relation can not be set to "(no selection)" when in completer mode

[RobinRots]

Describe the bug

When the value relation widget is set up in completer mode there is no simple way of setting a value back to "(no selection)". You can work around it by using the expression-box of the attribute table, thus setting values to NULL, but it should be as intuitive as when not in completer mode (then you can just select "(no selection)" in the drop-down). The problem is probably that deleting the value sets it back to a no-text-value '' but not to NULL. Probably "(no selection)" is only triggered when the value is set to NULL and not when it's ''.

How to Reproduce

see bugreport_completer_noselection.zip

1. Open the poject
2. Open the attribute table "value_relation_example" and put it in form-mode if it wouldn't be
3. Select a feature in the left panel and delete its value in the column "with_completer"
4. Select another feature in the left panel, then the ajusted feature again.
5. You now see that the value you just deleted is back again.

To make this more clear I have put it in a gif:

QGIS and OS versions

QGIS version	3.16.0-Hannover	QGIS code revision	43b64b1
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	3.1.4	Running against GDAL/OGR	3.1.4
Compiled against GEOS	3.8.1-CAPI-1.13.3	Running against GEOS	3.8.1-CAPI-1.13.3
Compiled against SQLite	3.29.0	Running against SQLite	3.29.0
PostgreSQL Client Version	11.5	SpatiaLite Version	4.3.0
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	6.3.2	Running against PROJ	Rel. 6.3.2, May 1st, 2020
OS Version	Windows 10 (10.0)
Active python plugins	autoSaver; geopunt4Qgis; OSMDownloader; qfieldsync; qgis-maptiler-plugin; quick_map_services; SelectWithin; splitmultipart; SpreadsheetLayers; db_manager; MetaSearch; processing

Additional context

I was testing whether the bugfix for bug #38801 solved my issue and found out it only partly does so. When the value relation widget is set up without completer functionality, it works fine. Thanks for that!
Also, see feature request #39677.

[gioman]

B/ the feature request

@RobinRots must be filed in a separate ticket.

[RobinRots]

B/ the feature request

@RobinRots must be filed in a separate ticket.

Done! Feature request deleted from this bug report and transferred to #39677

[Pedro-Murteira]

Still valid on QGIS 3.22.3. (Windows 10)