The size of A is larger or equal to m+n.
I believe that an empty A should be expressed like
//if n is 7
So basically, there is no empty A, is that right?
That's not quite correct, I think. Technically, if both m and n are 0, then A would be empty. However, I'm pretty sure that isn't a test case.
The inputs are representations of the starting arrays, not the data types. They only show the first m elements of A, and the first n elements of B (which is all of it). If you were to traverse the array A, you would find the actual array does have exactly m+n elements, but "array" A, which is shown in the input, only has m, so will often be shown as empty while it has n elements in reality.