Suppress bnd warnings for unused imports/exports (#11513)

Suppress bnd warnings for unused imports/exports (#11513)

Ignores warnings like:

```
Warning:  /home/runner/work/openhab-addons/openhab-addons/bom/runtime-index/pom.xml [0:0]: Unused Export-Package instructions: [org.openhab.*]
Warning:  /home/runner/work/openhab-addons/openhab-addons/bom/runtime-index/pom.xml [0:0]: Unused Import-Package instructions: [io.swagger.v3.oas.annotations.*,
```

These are safe to ignore because the import/export packages are globally defined and not every bundle imports/exports all these packages.

The `skipIfEmpty` configuration furthermore prevents warnings when the bnd-maven-plugin runs on projects that don't have any code like BOMs.

More important compiler/SAT warnings standout more when there are there are fewer useless warnings.

Signed-off-by: Wouter Born <github@maindrain.net>