compiler: Add test for length of empty arrays [nfc]

This makes sure we are actually emitting this as an 1D array
(like NumPy does).

artiq/test/lit/integration/array.py

@@ -6,5 +6,10 @@ assert len(ary) == 3
 # FIXME: Implement ndarray indexing
 # assert [x*x for x in ary] == [1, 4, 9]
 
+# Reassign to an existing value to disambiguate type of empty array.
+empty_array = array([1])
+empty_array = array([])
+assert len(empty_array) == 0
+
 matrix = array([[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]])
 assert len(matrix) == 2