numpy::from_data calls vector::front and crashes (or has UB) when called with a scalar object

In ndarray.cpp, the function `from_data_impl` calls `&shape.front()` and `&strides.front()` for possibly empty vectors. This is, at best, undefined behavior. At worst, it triggers a crash (observed on Redhat/rocky 8&9, where I'm assuming boost::python packages were built with `-D_GLIBCXX_DEBUG`).

RECOMMEND: change `&vec.front()` to `vec.data()` to avoid UB and potential crashes.

Start in ndarray.cpp at from_data_impl, where shape.front() and strides.front() are used for potentially empty vectors. Reproduce or inspect the scalar-object path and verify that it no longer performs invalid access or crashes; run the repository’s relevant existing tests if available. Done means scalar from_data calls work without undefined behavior or the reported crash.

Scritto dal modello di indicizzazione a partire dal testo della issue.