ceph-volume: address mypy errors in disk.py

typical error:

```
ceph_volume/util/disk.py:1374: error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "str")  [assignment]
```

This commits addresses it.

Signed-off-by: Guillaume Abrioux <[email protected]>

Author: guits

src/ceph-volume/ceph_volume/util/disk.py

@@ -1370,8 +1370,8 @@ def slashed_path(self) -> str:
         """
         result: str = self.path
         if self.is_lvm:
-            vg: str = self.environment.get('DM_VG_NAME')
-            lv: str = self.environment.get('DM_LV_NAME')
+            vg: str = self.environment.get('DM_VG_NAME', '')
+            lv: str = self.environment.get('DM_LV_NAME', '')
             result = f'/dev/{vg}/{lv}'
         return result
 
@@ -1385,6 +1385,6 @@ def dashed_path(self) -> str:
         """
         result: str = self.path
         if self.is_lvm:
-            name: str = self.environment.get('DM_NAME')
+            name: str = self.environment.get('DM_NAME', '')
             result = f'/dev/mapper/{name}'
         return result