docs: add note

Signed-off-by: Athan <kgryte@gmail.com>

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/gjoin/README.md

@@ -117,7 +117,8 @@ var str = gjoin.ndarray( 3, '|', x, 1, x.length-3 );
 
 ## Notes
 
--   If `N <= 0`, the function returns an empty string.
+-   If `N <= 0`, both functions return an empty string.
+-   If an array element is either `null` or `undefined`, both functions will serialize the element as an empty string.
 -   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/base/accessor`][@stdlib/array/base/accessor]).
 
 </section>