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

This reverts commit 7f9470cda390ee61c0fc71491d0c649fbf6e02dd.

Change-Id: I97e2859903fbc4f1e6f0824269ae88b23f63e032
diff --git a/tools/releasetools/sparse_img.py b/tools/releasetools/sparse_img.py
index 59d2b71..7574747 100644
--- a/tools/releasetools/sparse_img.py
+++ b/tools/releasetools/sparse_img.py
@@ -204,12 +204,8 @@
           nonzero_blocks.append(b)
           nonzero_blocks.append(b+1)
 
-    assert zero_blocks or nonzero_blocks
-
-    if zero_blocks:
-      out["__ZERO"] = rangelib.RangeSet(data=zero_blocks)
-    if nonzero_blocks:
-      out["__NONZERO"] = rangelib.RangeSet(data=nonzero_blocks)
+    out["__ZERO"] = RangeSet(data=zero_blocks)
+    out["__NONZERO"] = RangeSet(data=nonzero_blocks)
 
   def ResetFileMap(self):
     """Throw away the file map and treat the entire image as