fix missing artifact resolver for the bom-resolution

[balrok]

This issue was introduced with PR #287.
When calling the pomReader, the resolver was not always set and caused several (non-critical) exceptions in the log. Like:

Failed to retrieve artifacts for [group:com.google.code.gson, name:gson-parent, version:2.10.1, ext:pom]
java.lang.NullPointerException: Cannot invoke method resolveArtifacts() on null object


Failed to retrieve artifacts for [group:com.fasterxml.jackson, name:jackson-parent, version:2.16, ext:pom]
java.lang.NullPointerException: Cannot invoke method resolveArtifacts() on null object

Failed to retrieve artifacts for [group:software.amazon.awssdk, name:aws-sdk-java-pom, version:2.23.15, ext:pom]
java.lang.NullPointerException: Cannot invoke method resolveArtifacts() on null object

Those come frome the parents of the resolved bom-poms. This was due to having no resolver initialized.

After the change, it could resolved jackson-parent in the test and also added its license to the result. That's why the test changed slightly.