ARROW-80: Handle len call for pre-init arrays

Author: Uwe L. Korn <uwelk@xhochy.com>

Closes #45 from xhochy/arrow-80 and squashes the following commits:

d9a1160 [Uwe L. Korn] Add unit test for repr on pre-init Array
6208d7d [Uwe L. Korn] ARROW-80: Handle len call for pre-init arrays

python/pyarrow/array.pyx

@@ -67,7 +67,10 @@ cdef class Array:
         return '{0}\n{1}'.format(type_format, values)
 
     def __len__(self):
-        return self.sp_array.get().length()
+        if self.sp_array.get():
+            return self.sp_array.get().length()
+        else:
+            return 0
 
     def isnull(self):
         raise NotImplemented

python/pyarrow/tests/test_array.py

@@ -22,6 +22,10 @@
 
 class TestArrayAPI(unittest.TestCase):
 
+    def test_repr_on_pre_init_array(self):
+        arr = pyarrow.array.Array()
+        assert len(repr(arr)) > 0
+
     def test_getitem_NA(self):
         arr = pyarrow.from_pylist([1, None, 2])
         assert arr[1] is pyarrow.NA