fix(pg-codegen): read unqualified on an empty where, refuse an empty write filter

[pyramation]

Summary

A call site that qualifies a read with a conditional key column — the shape every scoped module read takes — states an empty filter when the scope records no key:

await db.identityProviders.findMany({
  where: module.entityField ? { [module.entityField]: frame.keyValue } : {},
  ...
});

The generated client passed that straight to QueryBuilder.where({}), which throws Empty filter object., so the global-scope path of every such read failed at runtime rather than reading unqualified.

The client now distinguishes "no predicate" from "no rows asked for":

-  if (args.where) query.where(this.filter(args.where));
+  const predicate = this.predicate(args.where);   // undefined when the filter has no keys
+  if (predicate) query.where(predicate);

and because the same emptiness on a write would silently mean every row, update/delete route through required(where, op) and throw instead:

users.update: refusing an empty where filter, which would match every row

findMany/findFirst/count read unqualified; update/delete throw. Two live round-trip tests cover both, and the committed __fixtures__/generated/client.ts is regenerated.

Link to Devin session: https://app.devin.ai/sessions/b1fb8d90ec0a4cd1bd95e3c77c6f6680
Requested by: @pyramation

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