Skip POM validation for unpublished projects

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.

Fixes and closes apple/pkl#215

Signed-off-by: Sam Gammon <sam@elide.ventures>

buildSrc/src/main/kotlin/pklPublishLibrary.gradle.kts

@@ -50,6 +50,9 @@ publishing {
 }
 
 val validatePom by tasks.registering {
+  if (tasks.findByName("generatePomFileForLibraryPublication") == null) {
+    return@registering
+  }
   val generatePomFileForLibraryPublication by tasks.existing(GenerateMavenPom::class)
   val outputFile = file("$buildDir/validatePom") // dummy output to satisfy up-to-date check