Fix: Assert When Using Shader Builder

Depending on the compiler shader builder could assert due to an
uninitialized attribute. This PR initializes the attribute.

Pull Request: https://projects.blender.org/blender/blender/pulls/116034

source/blender/gpu/intern/gpu_shader_builder.cc

@@ -26,7 +26,7 @@ namespace blender::gpu::shader_builder {
 class ShaderBuilder {
  private:
   GHOST_SystemHandle ghost_system_;
-  GHOST_ContextHandle ghost_context_;
+  GHOST_ContextHandle ghost_context_ = nullptr;
   GPUContext *gpu_context_ = nullptr;
 
  public: