if empty trange1, ptr is nullptr using Eigen3.5.x but 0 using Eigen3.4.x

Author: EricaCMitchell

src/TiledArray/tensor/tensor.h

@@ -185,7 +185,14 @@ class Tensor {
       : range_(range), nbatch_(nbatch) {
     size_t size = range_.volume() * nbatch;
     allocator_type allocator;
+#if EIGEN_HAS_MAJOR_VERSION_5
+    // Eigen 3.5.x returns nullptr if size=0
+    auto* ptr = (!size && range_.rank())
+                    ? static_cast<allocator_type::pointer>(std::malloc(size))
+                    : allocator.allocate(size);
+#else
     auto* ptr = allocator.allocate(size);
+#endif
     // default construct elements of data only if can have any effect ...
     if constexpr (!std::is_trivially_default_constructible_v<T>) {
       // .. and requested
@@ -1186,10 +1193,10 @@ class Tensor {
     bool empty = this->empty();
     auto range = this->range_;
     auto nbatch = this->nbatch_;
-    ar & empty;
+    ar& empty;
     if (!empty) {
-      ar & range;
-      ar & nbatch;
+      ar& range;
+      ar& nbatch;
       if constexpr (madness::is_input_archive_v<Archive>) {
         *this = Tensor(std::move(range), nbatch, default_construct{true});
       }