Give warning instead of error for wrong mongo URI (#2198)

Quick fix for issue #2197 By raising a warning instead of an error optimade should still work. I leave it up to @ml-evs to make a more permanent solution. 
Removed needless check for self.mongo_database

optimade/server/config.py

@@ -490,15 +490,17 @@ def align_mongo_uri_and_mongo_database(self) -> "ServerConfig":
 
         """
         if self.database_backend == SupportedBackend.MONGODB:
-            if self.mongo_uri and self.mongo_database:
+            if self.mongo_uri:
                 import pymongo.uri_parser
 
                 if not self.mongo_uri.startswith(
                     "mongodb://"
                 ) or self.mongo_uri.startswith("mongodb+srv://"):
                     self.mongo_uri = f"mongodb://{self.mongo_uri}"
 
-                uri: dict[str, Any] = pymongo.uri_parser.parse_uri(self.mongo_uri)
+                uri: dict[str, Any] = pymongo.uri_parser.parse_uri(
+                    self.mongo_uri, warn=True
+                )
                 if uri.get("database"):
                     self.mongo_database = uri["database"]