* BUG: Fortify object buffers against included NULLs

While NumPy tends to not actively create object buffers initialized
only with NULL (rather than filled with None), at least older versions
of NumPy did do that.  And NumPy guards against this.

This guards against embedded NULLs in object buffers interpreting
a NULL as None (and anticipating a NULL value also when setting
the buffer for reference count purposes).

Closes gh-4858