BIP 21: Clarify case-sensitivity

The "bitcoin:" URI scheme needs to be case-insensitive according to the RFC,
but the query parameters should be case-sensitive according to the bitcoin-qt
client implementation.
BIP 21: Allow empty addresses and query parameters

Empty addresses should be valid for compatibility with  BIP 72.

Empty query parameters should also be valid, since they are harmless,
and clients already accept them.

With these changes, bitcoin:?& would be a valid (but useless) URI.