make `env` optional when all env vars are optional (#54768)

When a component declares only optional env vars, the `env` property in `UseOptions` is now also optional (but can still be passed). Previously, `env` was only omittable when a component declared *no* env vars at all. This is achieved by checking whether any *required* env keys exist (i.e., those not wrapped in `v.optional(...)`) rather than checking whether any env keys exist at all.

GitOrigin-RevId: 34097363252891a74fab4d4d4699d36c75dbb759