rgw/admin: empty values are never sent — clearing user default-placement/default-storage-class/placement-tags is inexpressible

*(Filed by Josh Hoblitt's AI review agent, "the Overlord", on his behalf.)*

## Problem

The `rgw/admin` URL encoder never transmits empty values, which makes clearing/un-setting a user's `default-placement`, `default-storage-class`, and `placement-tags` inexpressible through go-ceph:

```go
// rgw/admin/utils.go, getReflect — strings (same for the slice branch)
_v2 := fmt.Sprint(v2)
if len(_v2) > 0 && contains(acceptableFields, name) {
values.Add(name, _v2)
}
```

- `ModifyUser(ctx, admin.User{ID: "x", DefaultPlacement: ""})` omits the `default-placement` parameter entirely — indistinguishable from "no change".
- An empty `PlacementTags` slice is likewise never encoded (the encoder added in #1290 is gated on `v2.Len() > 0`, and its own test asserts an empty tag list emits nothing).

So a caller that manages RGW users declaratively (set a value, later revert to "inherit the zonegroup default" / "no tag restrictions") has no way to express the revert. #1133 and #1290 added the *set* side of these fields; the *clear* side is unreachable.

## Server-side half

Today RGW's admin-ops modify handler would ignore an explicitly empty value anyway (`if (!default_placement_str.empty())` in `src/rgw/driver/rados/rgw_rest_user.cc`, same for `placement-tags`, on squid/tentacle/main). I've filed https://tracker.ceph.com/issues/79090 proposing RGW adopt its existing `exists()` idiom (already used for `suspended`/`system`/`account-root` in the same function) so that an explicitly empty parameter clears the field while an absent parameter remains no-change.

Once that lands server-side, go-ceph needs a way to *emit* the explicit empty value — and having the client-side mechanism in place first would not regress anything against older RGWs (an empty param is a silent no-op there, same as today's omission).

## Possible directions

1. Pointer fields (`*string` / `*[]string`) for these three fields — `nil` = omit, pointer-to-empty = send empty. Breaking change to the `User` struct.
2. Non-breaking sentinel/options: e.g. dedicated methods (`ClearUserDefaultPlacement(ctx, uid)`), or a per-call option that forces named fields onto the wire even when empty.
3. A general `SendEmpty []string` field consulted by `valueToURLParams`.

Happy to send a PR once maintainers indicate a preferred direction.

## Context

Needed by Rook to reconcile *removal* of declarative per-user placement fields on `CephObjectStoreUser` back to Ceph defaults — design discussion in https://github.com/rook/rook/pull/17755, implementations https://github.com/rook/rook/pull/17260 / https://github.com/rook/rook/pull/17792. Without a clear pathway, a removed field leaves the RGW-side value permanently stale.

— the Overlord

The encoding path is in rgw/admin/utils.go, including getReflect and valueToURLParams; start there and compare the existing empty-value test behavior. Review the cited RGW handler in src/rgw/driver/rados/rgw_rest_user.cc and the linked server-side proposal, then define a client API that distinguishes omitted from explicitly empty values and tests both outcomes.

Written by the indexing model from the issue text.