fix(extensions): pass over extension top-matter

the geometry extension file has some additional top-matter that we
weren't handling gracefully.

ibis_substrait/compiler/mapping.py

@@ -174,7 +174,7 @@ def castable(self) -> None:
 
 
 def _parse_func(entry: Mapping[str, Any]) -> Iterator[FunctionEntry]:
-    for impl in entry["impls"]:
+    for impl in entry.get("impls", []):
         sf = FunctionEntry(entry["name"])
         sf.parse(impl)