Align_image_stack has problem with these images. There are only some very small areas which are in focus and the out-of-focus area have nearly no structure.
So the found control points are of bad quality. As a consequence after control point pruning the reference image has no control points. When you now optimize
with hfov (-m switch) the hfov of the images is optimized to zero (but the hfov of the reference is not optimized and stays high).
The zero hfov results now in the empty output.
I fear these images are not good suited for align_image_stack. (For debug you can output a pto file (-p switch) and inspect the pto file in Hugin.)
Maybe playing with the parameters -t and/or --corr can help in this case. But don't expect wonders.
Thomas