define_from_variant: return an empty string for non-existing variants (#27503)

This permits to use conditional variants without a lot of boilerplate.

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

@@ -0,0 +1,9 @@
+# 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 CmakeConditionalVariantsTest(CMakePackage):
+    homepage = "https://dev.null"
+    version('1.0')
+    variant('example', default=True, description='nope', when='@2.0:')