C# node does not return empty proof enymore in case if path is bad. C#
node also throws an exception on bad Put.
Our node does not return an error on delete if the key is empty.
Allow it for (*Trie).Put. And distinguish empty value and nil value for
(*Trie).PutBatch, because batch is already capable of handling both nil
and empty value. For (*Trie).PutBatch putting nil value means deletion,
while putting empty value means just putting LeafNode with an empty
value.
