[API] Fix nHasDragDropPayload SIGSEGV when payload doesn't exist (#155)

`getDragDropPayload()` functions should simply return null when
the payload doesn't exist.

Currently, DearImGui will return NULL from `ImGui::GetDragDropPayload()`
when the payload does not exist. This causes a null dereference
since the nHasDragDropPayload function performs no NULL check

imgui-binding/src/main/java/imgui/ImGui.java

@@ -5118,11 +5118,13 @@ public class ImGui {
     }
 
     private static native boolean nHasDragDropPayload(); /*
-        return ImGui::GetDragDropPayload()->Data != NULL;
+        const ImGuiPayload* payload = ImGui::GetDragDropPayload();
+        return payload != NULL && payload->Data != NULL;
     */
 
     private static native boolean nHasDragDropPayload(String dataType); /*
-        return ImGui::GetDragDropPayload()->IsDataType(dataType);
+        const ImGuiPayload* payload = ImGui::GetDragDropPayload();
+        return payload != NULL && payload->IsDataType(dataType);
     */
 
     /**