Fix a crash when there's no available campaigns

twitch.py

@@ -650,9 +650,13 @@ class Twitch:
     async def fetch_inventory(self) -> None:
         response = await self.gql_request(GQL_OPERATIONS["Inventory"])
         inventory = response["data"]["currentUser"]["inventory"]
-        self.inventory = [
-            DropsCampaign(self, data) for data in inventory["dropCampaignsInProgress"]
-        ]
+        if inventory["dropCampaignsInProgress"]:  # None with no available campaigns
+            self.inventory = [
+                DropsCampaign(self, data) for data in inventory["dropCampaignsInProgress"]
+            ]
+        else:
+            # can happen if the user has no active campaigns yet
+            self.inventory = []
 
     def get_drop(self, drop_id: str) -> Optional[TimedDrop]:
         for campaign in self.inventory: