bug: fix possible missing models.

pyasic/miners/backends/bosminer.py

@@ -370,7 +370,9 @@ class BOSMiner(BaseMiner):
             return result.upper().strip()
 
     async def get_model(self) -> Optional[str]:
-        return self.model + " (BOS)"
+        if self.model is not None:
+            return self.model + " (BOS)"
+        return "? (BOS)"
 
     async def get_version(
         self, api_version: dict = None, graphql_version: dict = None

pyasic/miners/backends/hiveon.py

@@ -26,4 +26,6 @@ class Hiveon(BMMiner):
         self.api_type = "Hiveon"
 
     async def get_model(self) -> Optional[str]:
-        return self.model + " (Hiveon)"
+        if self.model is not None:
+            return self.model + " (Hiveon)"
+        return "? (Hiveon)"

pyasic/miners/backends/vnish.py

@@ -58,7 +58,9 @@ class VNish(BMMiner):
         self.data_locations = VNISH_DATA_LOC
 
     async def get_model(self) -> Optional[str]:
-        return self.model + " (VNISH)"
+        if self.model is not None:
+            return self.model + " (VNish)"
+        return "? (VNish)"
 
     async def restart_backend(self) -> bool:
         data = await self.web.restart_vnish()