Grammar for Secret Storage key ID

**Link to problem area**: https://spec.matrix.org/v1.18/client-server-api/#storage

**Issue**
It's not explicitly stated whether the key ID used to identify Secret Storage keys may be an empty string.

At least in the JS-SDK, an empty key ID is treated as an undefined key ([source](https://github.com/matrix-org/matrix-js-sdk/blob/v41.3.0/src/secret-storage.ts#L465)), which is reasonable. Nonetheless, the spec may as well take a stance on it, as technically speaking, there is nothing that prevents an empty string from being a usable key ID.

The Secret Storage key ID is used in the following places:
- The `m.secret_storage.key.[key ID]` account data type
- The value of the `key` parameter in the `m.secret_storage.default_key` account data object
- The keys of the `encrypted` map in encrypted account data objects

Start with the Secret Storage section of the Matrix Client-Server API specification linked in the issue, then locate the definitions for m.secret_storage.key.[key ID], m.secret_storage.default_key, and encrypted account data. Clarify in the specification whether an empty key ID is allowed or disallowed, and ensure the wording is consistent across all three uses.

Written by the indexing model from the issue text.