Include class name for warnings

[cymed]

Describe the bug
In qgepqwat2ili, missing mandatory values are logged with the warning
"WARNING A mandatory value was null. It will be cast to a blank string, and probably cause validation errors"
yet there is no information on which layer is affected.

To Reproduce
Exact steps to reproduce the behavior:

1. export an Interlis from a database with errors
2. See error
3. Check logs

Expected behavior
The warning should be extended to "WARNING A mandatory value (attribute name) in the layer (layer_name) was null. It will be cast to a blank string, and probably cause validation errors"

Screenshots / data
dump can be sent by email upon request

Desktop (please complete the following information):

• QGEP plugin version 20230411 test vsa-dss
• QGEP datamodel version 1.6.0
• QGEP project file version 1.6.1
• QGIS Version 3.28.5 LTR
• sqlalchemy Version 1.3.24
• geoalchemy2 Version 0.11.1

[sjib]

@cymed The function null_to_empty is only used for the attribut "identifier/Bezeichnung" as this is the only attribute besides obj_id that is MANDATORY in INTERLIS - see export.py

I am currently testing a new check to see if there are missing identifiers - see https://github.com/QGEP/qgepqwat2ili/pull/108/files#r1166864199

[cymed]

NULL entries on ws_status can also lead to problems, even if the field is not mandatory.

[sjib]

@cymed Can you post an example?