IVGCVSW-3307 Don't assume TensorInfo::Map() can be called before Execute()

Change-Id: I445c69d2e99d8c93622e739af61f721e61b0f90f
Signed-off-by: Matthew Bentham <Matthew.Bentham@arm.com>
diff --git a/src/backends/reference/workloads/Decoders.hpp b/src/backends/reference/workloads/Decoders.hpp
index f5ec906..793e550 100644
--- a/src/backends/reference/workloads/Decoders.hpp
+++ b/src/backends/reference/workloads/Decoders.hpp
@@ -13,7 +13,7 @@
 {
 
 template<typename T>
-inline std::unique_ptr<Decoder<T>> MakeDecoder(const TensorInfo& info, const void* data);
+inline std::unique_ptr<Decoder<T>> MakeDecoder(const TensorInfo& info, const void* data = nullptr);
 
 template<>
 inline std::unique_ptr<Decoder<float>> MakeDecoder(const TensorInfo& info, const void* data)