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 = {}
    -------------------------------------------------------------------^

1 files changed, 10 insertions, 1 deletions

diff --git a/src/iso19111/coordinateoperation.cpp b/src/iso19111/coordinateoperation.cpp
index d5bcfa84..0eef9954 100644
--- a/src/iso19111/coordinateoperation.cpp
+++ b/src/iso19111/coordinateoperation.cpp
@@ -552,7 +552,16 @@ struct CoordinateOperation::Private {
 
 // ---------------------------------------------------------------------------
 
-GridDescription::GridDescription() = default;
+GridDescription::GridDescription():
+    shortName{},
+    fullName{},
+    packageName{},
+    url{},
+    directDownload(false),
+    openLicense(false),
+    available(false)
+{}
+
 
 GridDescription::~GridDescription() = default;