Fixes #68 - Adds checks for zero length arrays/slices.

[haasanen]

Fixes #68

The problem:

The following code (or any other array/slice type!):

var t []uuid.UUID
tx.QueryRow(ctx, `select '{}'::uuid[]`).Scan(&t)

Causes the src struct in the AssignTo function to have Elements and Dimensions set as nil, but Status as Present.

The proposed solution:

• Adds checks for nil or zero length elements or dimensions in the AssignTo function where the src is marked as Present, but the Elements or Dimensions are nil.
• Returns immediately for nil and zero length arrays and slices.
• Adds the above issue as test cases for all array types.
• Only 4 lines of new code in typed_array.go.erb + test cases.

[jackc]

Thanks!