Fix to_bjdata() emitting unparsable output when _ArraySize_ is not an array (#5455)

* Fix to_bjdata() emitting unparsable output when _ArraySize_ is not an array

write_bjdata_ndarray() never checked that _ArraySize_ is an array. The shape
is written verbatim as the header length, so a null shape emitted 'Z' and an
object shape emitted '{' after the '#', neither of which from_bjdata()
accepts, and the round-trip guarantee in the BJData docs was broken.

Both slipped through the existing validation: for null, empty() is true so
the element count starts at 0 and the per-dimension loop never runs, and for
an object the loop walks its values, which can satisfy the non-negative
integer check. When _ArrayData_ then matched that count, the writer took the
ndarray path.

Require the shape to be an array, so anything else falls back to a plain
object encoding that round-trips, as the fallback rule in the docs already
specifies.

Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>

* Document that _ArraySize_ must be an array in the ndarray requirements

The list at bjdata.md is the exhaustive set of conditions for the ndarray
encoding, but it only implied this one through 'every entry of'.

Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>

---------

Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>

include/nlohmann/detail/output/binary_writer.hpp

@@ -1668,6 +1668,15 @@ class binary_writer
         CharType dtype = it->second;
 
         key = "_ArraySize_";
+        // the dimensions are written verbatim as the header length below, so a
+        // value that is not an array cannot produce a valid one: null emits 'Z'
+        // and an object emits '{', neither of which a reader accepts after '#'.
+        // Such an object is not a valid ndarray and falls back to a plain object.
+        if (!value.at(key).is_array())
+        {
+            return true;
+        }
+
         std::size_t len = (value.at(key).empty() ? 0 : 1);
         for (const auto& el : value.at(key))
         {