am 1aa1af0e: sparse_img.py: Don\'t write empty map groups.

* commit '1aa1af0e997699bc025e4f0a754692068328d7e6':
  sparse_img.py: Don't write empty map groups.

tools/releasetools/sparse_img.py

@@ -201,7 +201,11 @@ class SparseImage(object):
           nonzero_blocks.append(b)
           nonzero_blocks.append(b+1)
 
+    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):