Fix nbtrans overrides validation schema

[ryaplots]

Summary

References this https://github.com/TheThingsIndustries/lorawan-stack-support/issues/1071#issuecomment-2191996617

Changes

• Fix validation schema to allow empty values

Testing

Steps

Update NbTrans values using the below CLI commands:

ttn-lw-cli dev set nb-trans-test eui-0004a30b001bc9bb --mac-settings.adr.mode.dynamic.overrides.data-rate-5.max-nb-trans 3
ttn-lw-cli dev set nb-trans-test eui-0004a30b001bc9bb --mac-settings.adr.mode.dynamic.overrides.data-rate-3.min-nb-trans 2
ttn-lw-cli dev set nb-trans-test eui-0004a30b001bc9bb --mac-settings.adr.mode.dynamic.overrides.data-rate-1.min-nb-trans 2 --mac-settings.adr.mode.dynamic.overrides.data-rate-5.max-nb-trans 3

Results

The network layer page should not crash:

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.