QGIS inserts empty array into nullable array fields by default when creating new features in a PostGIS layer

[peetw]

Describe the bug

Given the following PostGIS table schema:

CREATE TABLE test
(
    id SERIAL PRIMARY KEY,
    geom GEOMETRY(LineString, 4326) NOT NULL,
    array INTEGER[]
)

QGIS will insert an empty PostgreSQL array (i.e. {}) into the array field by default when creating new features whilst editing the test PostGIS layer. However, since the array field is nullable (i.e. it is not marked NOT NULL), I would expect QGIS to enter NULL into the array field by default (as this would be the default value if editing the table using other software).

How to Reproduce

See above.

QGIS and OS versions

QGIS version	3.10.7-A Coruña	QGIS code revision	7b4ca4c
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	3.0.4	Running against GDAL/OGR	3.0.4
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 Server 2016 (10.0)
Active python plugins	profiletool; quick_map_services; valuetool; db_manager; MetaSearch; processing

Additional context

[peetw]

Happy to help with this one if needed, but not sure where would be a good place to start? I had a quick search through the src files for the source code for the new feature save dialog, but couldn't find anything.

[rouault]

This is not an issue with the Postgres provider, but with the editor widget of an integer list that returns a 0-sized array, and not a NULL value

[peetw]

@rouault is that something that could be changed without breaking other components? Or is this likely to be a larger change (the integer list editor widget sounds fairly fundamental)? Would this issue also affect non-integer (e.g. real/float) lists?