cellid_import: fix TypeError when --mcc or --mnc are unspecified

`int(None)` (in the case these arguments are unspecified) gives a TypeError.

```
$ cellid-ols-import -o cell_towers.db cell_towers.csv
[2024-06-21 13:34:48.169] DEBUG: Database version: 1
Traceback (most recent call last):
  File "cellid-ols-import", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File "ols/resolver/cellid/cellid_import.py", line 43, in main
    mcc = int_or_none(args.MCC)
          ^^^^^^^^^^^^^^^^^^^^^
  File "ols/resolver/cellid/cellid_import.py", line 18, in int_or_none
    return int(s)
           ^^^^^^
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'
```

ols/resolver/cellid/cellid_import.py

@@ -16,7 +16,7 @@ log = logging.getLogger(__name__)
 def int_or_none(s):
     try:
         return int(s)
-    except ValueError:
+    except (TypeError, ValueError):
         return None