feat(duckdb): allow to use named in-memory db (#9241)

Allows to use a named in-memory database with the DuckDB backend.

* Resolves #9240

ibis/backends/duckdb/__init__.py

@@ -454,10 +454,8 @@ def do_connect(
         <ibis.backends.duckdb.Backend object at ...>
 
         """
-        if (
-            not isinstance(database, Path)
-            and database != ":memory:"
-            and not database.startswith(("md:", "motherduck:"))
+        if not isinstance(database, Path) and not database.startswith(
+            ("md:", "motherduck:", ":memory:")
         ):
             database = Path(database).absolute()