Add Null rendering backend

Doesn't implement any functions, currently not selectable or
configurable

src/core/renderer_null/renderer_null.cpp

@@ -0,0 +1,12 @@
+#include "renderer_null/renderer_null.hpp"
+
+RendererNull::RendererNull(GPU& gpu, const std::array<u32, regNum>& internalRegs) : Renderer(gpu, internalRegs) {}
+RendererNull::~RendererNull() {}
+
+void RendererNull::reset() {}
+void RendererNull::display() {}
+void RendererNull::initGraphicsContext() {}
+void RendererNull::clearBuffer(u32 startAddress, u32 endAddress, u32 value, u32 control) {}
+void RendererNull::displayTransfer(u32 inputAddr, u32 outputAddr, u32 inputSize, u32 outputSize, u32 flags) {}
+void RendererNull::drawVertices(PICA::PrimType primType, std::span<const PICA::Vertex> vertices) {}
+void RendererNull::screenshot(const std::string& name) {}