fix: Change empty string to null in products subscription config

[svariant]

List of Changes

• Changed empty string to null in products subscription config

Motivation and Context

Readability and usage

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud