Build: Skip jar tests if jar disabled

The shadow plugin disables the jar task but we still attempted to extract
the jar to see if it had the right license and notice file. This skips
the extraction and those tests if the jar is built for any reason which
fixes projects that use the shadow plugin.

build.gradle

@@ -568,6 +568,7 @@ subprojects { project ->
       commandLine "${->new File(rootProject.compilerJavaHome, 'bin/jar')}",
           'xf', "${-> jarTask.outputs.files.singleFile}", 'META-INF/LICENSE.txt', 'META-INF/NOTICE.txt'
       workingDir destination
+      onlyIf {jarTask.enabled}
       doFirst {
         project.delete(destination)
         Files.createDirectories(destination)
@@ -576,6 +577,7 @@ subprojects { project ->
 
     final Task checkNotice = project.task("verify${jarTask.name.capitalize()}Notice") {
       dependsOn extract
+      onlyIf {jarTask.enabled}
       doLast {
         final List<String> noticeLines = Files.readAllLines(project.noticeFile.toPath())
         final Path noticePath = extract.destination.resolve('META-INF/NOTICE.txt')
@@ -586,6 +588,7 @@ subprojects { project ->
 
     final Task checkLicense = project.task("verify${jarTask.name.capitalize()}License") {
       dependsOn extract
+      onlyIf {jarTask.enabled}
       doLast {
         final List<String> licenseLines = Files.readAllLines(project.licenseFile.toPath())
         final Path licensePath = extract.destination.resolve('META-INF/LICENSE.txt')