Fixed empty structs generating illegal constructor in C++.

Change-Id: I60418d597b497c3cfa655ad76b3128351e9139f2

src/idl_gen_cpp.cpp

@@ -1354,6 +1354,7 @@ class CppGenerator : public BaseGenerator {
     code += struct_def.name + ")); }\n";
 
     // Generate a constructor that takes all fields as arguments.
+    if (struct_def.fields.vec.size()) {
       code += "  " + struct_def.name + "(";
       for (auto it = struct_def.fields.vec.begin();
            it != struct_def.fields.vec.end(); ++it) {
@@ -1378,7 +1379,6 @@ class CppGenerator : public BaseGenerator {
         }
         GenPadding(field, code, padding_id, PaddingInitializer);
       }
-
       code += " {";
       padding_id = 0;
       for (auto it = struct_def.fields.vec.begin();
@@ -1387,6 +1387,7 @@ class CppGenerator : public BaseGenerator {
         GenPadding(field, code, padding_id, PaddingDeclaration);
       }
       code += " }\n\n";
+    }
 
     // Generate accessor methods of the form:
     // type name() const { return flatbuffers::EndianScalar(name_); }