sparse_img.py: Don't write empty map groups.

In certain cases, we may have no mapping of ZERO or NONZERO blocks in
the image (such as squashfs at the moment). Don't write those groups if
they are empty.

Change-Id: I4a1f0105ada2e1573c3d6162457b83725e0d8971

tools/releasetools/sparse_img.py

@@ -204,8 +204,12 @@ class SparseImage(object):
           nonzero_blocks.append(b)
           nonzero_blocks.append(b+1)
 
-    out["__ZERO"] = RangeSet(data=zero_blocks)
-    out["__NONZERO"] = RangeSet(data=nonzero_blocks)
+    assert zero_blocks or nonzero_blocks
+
+    if zero_blocks:
+      out["__ZERO"] = RangeSet(data=zero_blocks)
+    if nonzero_blocks:
+      out["__NONZERO"] = RangeSet(data=nonzero_blocks)
 
   def ResetFileMap(self):
     """Throw away the file map and treat the entire image as