sepolicy: set conf.substitutions['releasever'] to empty str when releasever is None

[yizhao1]

For some distributions (e.g. Yocto) that do not provide system-release/distribution-release file, libdnf can not get releasever variable, causing conf.substitutions['releasever'] to not be set. This will cause 'sepolicy generate' command to fail with the following error on these distributions:

$ sepolicy generate --init /usr/local/bin/foo
Traceback (most recent call last):
File "/usr/bin/sepolicy", line 702, in
args.func(args)
File "/usr/bin/sepolicy", line 569, in generate
mypolicy.gen_writeable()
File "/usr/lib/python3.12/site-packages/sepolicy/generate.py", line 1302, in gen_writeable
self.__extract_rpms()
File "/usr/lib/python3.12/site-packages/sepolicy/generate.py", line 1268, in __extract_rpms
base.read_all_repos()
File "/usr/lib/python3.12/site-packages/dnf/base.py", line 554, in read_all_repos
for repo in reader:
^^^^^^
File "/usr/lib/python3.12/site-packages/dnf/conf/read.py", line 42, in iter
for r in self._get_repos(self.conf.config_file_path):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/site-packages/dnf/conf/read.py", line 109, in _get_repos
parser.setSubstitutions(substs)
File "/usr/lib/python3.12/site-packages/libdnf/conf.py", line 1643, in setSubstitutions
return _conf.ConfigParser_setSubstitutions(self, substitutions)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: in method 'ConfigParser_setSubstitutions', argument 2 of type 'std::map< std::string,std::string,std::less< std::string >,std::allocator< std::pair< std::string const,std::string > > > const &'

Set conf.substitutions['releasever'] to empty str if releasever is None.

[bachradsusi]

Hello, thanks for the patch!

In order to get the patch reviewed and accepted, it's necessary to send it to selinux@vger.kernel.org mailing list. The best way is to use git send-email command, e.g.:

 git send-email --from='Your Name <your@email.address>' --to=selinux@vger.kernel.org --smtp-server=my.smtp.server --confirm=auto -1

See https://github.com/SELinuxProject/selinux/blob/main/CONTRIBUTING.md