Fix doc generation with Breathe 4.12.0

Breathe 4.12.0 (as pulled by MacOSX builds such as
https://travis-ci.com/OSGeo/proj.4/jobs/185395222)
does not seem to like default initialization in documented C++ structs (regression/bug)

/Users/travis/build/OSGeo/proj.4/docs/source/development/reference/cpp/io.rst:6:Parsing of expression failed. Using fallback parser. Error was:
Error in postfix expression, expected primary expression or type.
If primary expression:
  Invalid definition: Expected identifier in nested name. [error at 67]
    std::string osgeo::proj::io::AuthorityFactory::CRSInfo::authName = {}
    -------------------------------------------------------------------^
If type:
  Invalid definition: Expected identifier in nested name. [error at 67]
    std::string osgeo::proj::io::AuthorityFactory::CRSInfo::authName = {}
    -------------------------------------------------------------------^

0 files changed, 0 insertions, 0 deletions