Ensure that SimpleShaderObjectData returns nullptr for empty buffer

This is a C++ technicality, and is not known to have caused any issue.

https://en.cppreference.com/w/cpp/container/vector/data says:
"If size() is ​0​, data() may or may not return a null pointer."

Thus, we check if the underlying std::vector is empty and then explicitly return nullptr
in this case.

src/rhi-shared.h

@@ -437,7 +437,7 @@ class SimpleShaderObjectData
 
     Index getCount() { return m_ordinaryData.size(); }
     void setCount(Index count) { m_ordinaryData.resize(count); }
-    uint8_t* getBuffer() { return m_ordinaryData.data(); }
+    uint8_t* getBuffer() { return m_ordinaryData.empty() ? nullptr : m_ordinaryData.data(); }
 
     /// Returns a StructuredBuffer resource view for GPU access into the buffer content.
     /// Creates a StructuredBuffer resource if it has not been created.