BLI: initialize MutableSpan in default constructor

source/blender/blenlib/BLI_span.hh

@@ -478,8 +478,8 @@ template<typename T> class MutableSpan {
   using size_type = int64_t;
 
  protected:
-  T *data_;
-  int64_t size_;
+  T *data_ = nullptr;
+  int64_t size_ = 0;
 
  public:
   constexpr MutableSpan() = default;