This has no effect on the correct code currently, but will help in
debugging when changing the serialized metadata for properties.
For example, if after some change an empty propertySignature extension
is written for Property (it's possible for PropertyDescriptor without
getter or setter and without backing field), we now won't try to read a
missing getter message when loading annotations on property getter. In
protobuf, reading a missing message will result not in an exception, but
in a "default" message being returned, which makes no sense in our case
because it would simply be read as a getter signature with both "name"
and "desc" equal to 0 (i.e. the first entry in the string table)
Alexander Udalov
b9594988ea