I have modified an old (classic) instance
by adding a new multilink property. hyperdb.py
then fails in set/get for this new property,
because although it knows the property it still
tries to get it from the dictionary, causing
a KeyError.

My understanding is that KeyError should only
be thrown for unknown, but not for empty
properties, therefore I have patched hyperdb.py.

Setting was affected because it tried to
compare against the existing entry. Normal
properties like String() still need the patch
for getting.