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, 7 insertions, 7 deletions

diff --git a/include/proj/coordinateoperation.hpp b/include/proj/coordinateoperation.hpp
index 0ff27203..3111a2a3 100644
--- a/include/proj/coordinateoperation.hpp
+++ b/include/proj/coordinateoperation.hpp
@@ -63,15 +63,15 @@ namespace operation {
 
 /** \brief Grid description */
 struct GridDescription {
-    std::string shortName{};   /**< Grid short filename */
-    std::string fullName{};    /**< Grid full path name (if found) */
-    std::string packageName{}; /**< Package name (or empty) */
-    std::string url{}; /**< Grid URL (if packageName is empty), or package
+    std::string shortName;   /**< Grid short filename */
+    std::string fullName;    /**< Grid full path name (if found) */
+    std::string packageName; /**< Package name (or empty) */
+    std::string url; /**< Grid URL (if packageName is empty), or package
                             URL (or empty) */
-    bool directDownload = false; /**< Whether url can be fetched directly. */
+    bool directDownload; /**< Whether url can be fetched directly. */
     /** Whether the grid is released with an open license. */
-    bool openLicense = false;
-    bool available = false; /**< Whether GRID is available. */
+    bool openLicense;
+    bool available; /**< Whether GRID is available. */
 
     //! @cond Doxygen_Suppress
     bool operator<(const GridDescription &other) const {