Fix crash on empty integer config values (#11)

config.get() now returns default (or 0) when an INT_KEY contains
an empty string, instead of crashing with ValueError.
Fixes settings like mqtt_port or booked_download being saved as ''.

Author: Dennis Braun

app/config.py

@@ -175,6 +175,8 @@ def get(self, key, default=None):
         if key in self._file_config:
             val = self._file_config[key]
             if key in INT_KEYS and not isinstance(val, int):
+                if val == "" or val is None:
+                    return default if default is not None else 0
                 return int(val)
             if key in HASH_KEYS:
                 # Return werkzeug hash as-is; legacy Fernet-encrypted values get decrypted