Skip POM validation for unpublished projects

[sgammon]

Summary

This small change fixes a bug introduced by the validatePom task within the pklPublishLibrary plugin. When running project-wide tasks like ./gradle tasks, projects which don't provide a POM fail the build, because the generatePomFileForLibraryPublication cannot be found, but it is expected to be present.

With this change applied, validatePom only runs if the task generatePomFileForLibraryPublication is present in a given project.

Fixes and closes apple/pkl#215