Fix: ASAN error with Span.slice_safe with empty span

Happened running io_usd_import test.

source/blender/blenlib/BLI_span.hh

@@ -157,7 +157,7 @@ template<typename T> class Span {
     BLI_assert(start >= 0);
     BLI_assert(size >= 0);
     const int64_t new_size = std::max<int64_t>(0, std::min(size, size_ - start));
-    return Span(data_ + start, new_size);
+    return Span(data_ ? data_ + start : nullptr, new_size);
   }
 
   constexpr Span slice_safe(IndexRange range) const