An array formed with Outer Product is empty (has no elements) if any one of the lists used is empty.
(⍳2) ∘.× ⍬ ∘.× (⍳4)
⍴ (⍳2) ∘.× ⍬ ∘.× (⍳4)
2 0 4
APL still remembers the shape!
+⌿ (⍳2) ∘.× ⍬ ∘.× (⍳4) ⍝ Shape 0 4: no rows
+⌿ +⌿ (⍳2) ∘.× ⍬ ∘.× (⍳4)
0 0 0 0