fix(slurm_ops): use correct type hints for environment variables getters

Environment variable getters can reuturn str or None, not just str.
None is returned if the environment variable is not defined in
the corresponding environment file.

Signed-off-by: Jason C. Nucciarone <[email protected]>

lib/charms/hpc_libs/v0/slurm_ops.py

@@ -912,9 +912,10 @@ def __init__(self, *args, **kwargs) -> None:
         self._env_manager = self._ops_manager.env_manager_for(_ServiceType.SACKD)
 
     @property
-    def config_server(self) -> str:
+    def config_server(self) -> str | None:
         """Get the configuration server address of this `sackd` node."""
-        return self._env_manager.get("SACKD_CONFIG_SERVER")
+        result = self._env_manager.get("SACKD_CONFIG_SERVER")
+        return None if result == "" else result
 
     @config_server.setter
     def config_server(self, addr: str) -> None:
@@ -971,7 +972,7 @@ def group(self) -> str:
         return "root"
 
     @property
-    def config_server(self) -> str:
+    def config_server(self) -> str | None:
         """Get the configuration server address of this `slurmd` node."""
         return self._env_manager.get("SLURMD_CONFIG_SERVER")
 
@@ -1000,7 +1001,7 @@ def __init__(self, *args, **kwargs) -> None:
         )
 
     @property
-    def mysql_unix_port(self) -> str:
+    def mysql_unix_port(self) -> str | None:
         """Get the URI of the unix socket `slurmdbd` uses to communicate with MySQL."""
         return self._env_manager.get("MYSQL_UNIX_PORT")