This is a fix for #1300

Now that externalization work better for nil or zero to null (done in #1194 as part of #617), we need to be able to internalize null values into nil or zero. Currently we get an exception because internalization isn't expecting a null value for a slice or array.

Internalization of an externalized array creates a copy making the value unassignable (see #1302). Until that is fixed, trying to internalize a null array will simply throw an exception. This is favorable over returning a zero value which will quietly hide the array problem and still not be assignable.