Fix segfault writing ascii .vtu files with implicit cell type arrays

Fix segfault writing ascii .vtu files with implicit cell type arrays

Forwarded: not-yet

NewIterator() returns nullptr for the implicit cell-type arrays used
since VTK 9.6. WriteAsciiData() dereferenced that nullptr via an
unconditional iter->Delete(). Guard it. Closes yade FTBFS.

Gbp-Pq: Name fix_xmlwriter_ascii_implicit_cellarray_segfault.patch