Make _enable_or_disable(...) return an empty array for conditional variants whose condition is not met (#27504)

var/spack/repos/builtin.mock/packages/autotools-conditional-variants-test/package.py

@@ -0,0 +1,11 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+class AutotoolsConditionalVariantsTest(AutotoolsPackage):
+    homepage = "https://www.example.com"
+    has_code = False
+    version('1.0')
+    variant('example', default=True, description='nope', when='@2.0:')