Don't crash when imported but DB url is not present

src/replit/database/__init__.py

@@ -276,4 +276,12 @@ class ReplitDb(dict):
         return f"<ReplitDb(db_url={self.db_url!r})>"
 
 
-db = ReplitDb(os.environ["REPLIT_DB_URL"])
+db_url = os.environ.get("REPLIT_DB_URL")
+if db_url:
+    db = ReplitDb(db_url)
+else:
+    print(
+        "Warning: REPLIT_DB_URL does not exist, are we running on repl.it? Database will not function.",
+        file=stderr,
+    )
+    db = None