Nasal Hash: add isNil wrapper

simgear/nasal/cppbind/NasalHash.cxx

@@ -130,4 +130,10 @@ namespace nasal
     return _keys;
   }
 
+  //----------------------------------------------------------------------------
+  bool Hash::isNil() const
+  {
+      return naIsNil(_hash);
+  }
+
 } // namespace nasal

simgear/nasal/cppbind/NasalHash.hxx

@@ -137,6 +137,8 @@ namespace nasal
        */
       int size() const;
 
+      bool isNil() const;
+
       /**
        * Get a list of all keys
        */