INTERLIS label export - no warning if no scale is selected

[sjib]

Describe the bug
Default selection of label export does not contain a scale. If no scale is selected export process is running through without warning

To Reproduce
Exact steps to reproduce the behavior:

1. Click on INTERLIS Export
2. Click on OK
3. Export is running thru
4. Labels are not exported

Expected behavior

• Do not run INTERLIS export if "Label export" is selected, but no scale is chosen.
• Show warning "Choose a least one label scale!" and do not start export process

Screenshots / data

Desktop (please complete the following information):

• QGEP plugin version 1.5.6
• QGEP datamodel version 1.5.5
• QGEP project file version
• QGIS Version 3.26.3
• OS Windows 10