"const Node Node::operator[](const Key& key) const" changed from
returning new empty node if the key was missing in 0.5.1 to returning
a shared 'zombie' node in 0.5.2 to resolve a memory leak.
(Specifically 1025f76df1 was where this
was introduced)
This caused some regressions where this 'zombie' object threw exceptions
in some functions where the 'empty' object would not.
This change fixes the Node::as(fallback) method (to return the
'fallback' instead of throwing an exception) and the
Node::begin()/Node::end() methods to return default-constructed
iterators (so begin() == end() in such cases) instead of another
exception.
b426fafff6