[ntcore] Use json_fwd (#5881)

Also fix ProtobufEntry construction.

ntcore/src/main/native/include/networktables/ProtobufTopic.h

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include <wpi/SmallVector.h>
+#include <wpi/json_fwd.h>
 #include <wpi/mutex.h>
 #include <wpi/protobuf/Protobuf.h>
 
@@ -21,10 +22,6 @@
 #include "networktables/Topic.h"
 #include "ntcore_cpp.h"
 
-namespace wpi {
-class json;
-}  // namespace wpi
-
 namespace nt {
 
 template <wpi::ProtobufSerializable T>
@@ -303,7 +300,7 @@ class ProtobufEntry final : public ProtobufSubscriber<T>,
    */
   ProtobufEntry(NT_Entry handle, wpi::ProtobufMessage<T> msg, T defaultValue)
       : ProtobufSubscriber<T>{handle, std::move(msg), std::move(defaultValue)},
-        ProtobufPublisher<T>{handle, {}} {}
+        ProtobufPublisher<T>{handle, wpi::ProtobufMessage<T>{}} {}
 
   /**
    * Determines if the native handle is valid.

ntcore/src/main/native/include/networktables/StructArrayTopic.h

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include <wpi/SmallVector.h>
+#include <wpi/json_fwd.h>
 #include <wpi/mutex.h>
 #include <wpi/struct/Struct.h>
 
@@ -21,10 +22,6 @@
 #include "networktables/Topic.h"
 #include "ntcore_cpp.h"
 
-namespace wpi {
-class json;
-}  // namespace wpi
-
 namespace nt {
 
 template <wpi::StructSerializable T>

ntcore/src/main/native/include/networktables/StructTopic.h

@@ -14,16 +14,13 @@
 #include <vector>
 
 #include <wpi/SmallVector.h>
+#include <wpi/json_fwd.h>
 #include <wpi/struct/Struct.h>
 
 #include "networktables/NetworkTableInstance.h"
 #include "networktables/Topic.h"
 #include "ntcore_cpp.h"
 
-namespace wpi {
-class json;
-}  // namespace wpi
-
 namespace nt {
 
 template <wpi::StructSerializable T>