Ensure we read the full TLV stream length when maybe-reading `None`

If we are reading an object that is `MaybeReadable` in a TLV stream
using `upgradable_required`, it may return early with `Ok(None)`.
In this case, it will not read any further TLVs from the TLV
stream. This is fine, except that we generally expect
`MaybeReadable` always consume the correct number of bytes for the
full object, even if it doesn't understand it.

This could pose a problem, for example, in cases where we're
reading a TLV-stream `MaybeReadable` object inside another
TLV-stream object. In that case, the `MaybeReadable` object may
return `Ok(None)` and not consume all the available bytes, causing
the outer TLV read to fail as the TLV length does not match.