Always define the path DNSSEC_OPENSSL_CONF

The variable was None by default and set to /etc/ipa/dnssec/openssl.cnf
for fedora only because the code is specific to the support of pkcs11
engine for bind. As a consequence ipa-backup had a "None" value in the
list of files to backup and failed on Exception.

ipa-backup code is able to handle missing files, and the code using
the pkcs11 engine is called only when NAMED_OPENSSL_ENGINE is set
(only in fedora so far). It is safe to always define a value for
DNSSEC_OPENSSL_CONF even on os where it does not exist.

The fix also improves the method used to verify that a path exists.

Fixes: https://pagure.io/freeipa/issue/8597
Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>

ipaserver/install/ipa_backup.py

@@ -626,7 +626,7 @@ class Backup(admintool.AdminTool):
     def file_backup(self, options):
 
         def verify_directories(dirs):
-            return [s for s in dirs if os.path.exists(s)]
+            return [s for s in dirs if s and os.path.exists(s)]
 
         self.tarfile = os.path.join(self.dir, 'files.tar')