VideoBackends/Null: Apply final to classes where applicable

These aren't intended to be further specialized, so we can make this
obvious with final.

Source/Core/VideoBackends/Null/PerfQuery.h

@@ -8,7 +8,7 @@
 
 namespace Null
 {
-class PerfQuery : public PerfQueryBase
+class PerfQuery final : public PerfQueryBase
 {
 public:
   void EnableQuery(PerfQueryGroup type) override {}

Source/Core/VideoBackends/Null/Render.h

@@ -8,7 +8,7 @@
 
 namespace Null
 {
-class Renderer : public ::Renderer
+class Renderer final : public ::Renderer
 {
 public:
   Renderer();

Source/Core/VideoBackends/Null/TextureCache.h

@@ -8,7 +8,7 @@
 
 namespace Null
 {
-class TextureCache : public TextureCacheBase
+class TextureCache final : public TextureCacheBase
 {
 protected:
   void CopyEFB(AbstractStagingTexture* dst, const EFBCopyParams& params, u32 native_width,

Source/Core/VideoBackends/Null/VertexManager.h

@@ -8,7 +8,7 @@
 
 namespace Null
 {
-class VertexManager : public VertexManagerBase
+class VertexManager final : public VertexManagerBase
 {
 public:
   VertexManager();

Source/Core/VideoBackends/Null/VideoBackend.h

@@ -8,7 +8,7 @@
 
 namespace Null
 {
-class VideoBackend : public VideoBackendBase
+class VideoBackend final : public VideoBackendBase
 {
   bool Initialize(const WindowSystemInfo& wsi) override;
   void Shutdown() override;