[pid2] Allow optional arrays

We have optional arrays, and it would be real neat if we could codegen
them.

This change codegens them.

Author: lhchavez

src/replit_river/codegen/client.py

@@ -629,6 +629,11 @@ def extract_props(tpe: RiverType) -> list[dict[str, RiverType]]:
                                                 for y in x[{repr(name)}]
                                             ]
                                             """.rstrip()
+                                                + (
+                                                    f" if x.get({repr(name)}) else None "
+                                                    if name not in prop.required
+                                                    else ""
+                                                )
                                             )
                                         )
                         else: