[subset] Don't produce empty BlueValues

OTS didn't like it.
diff --git a/Lib/fontTools/subset.py b/Lib/fontTools/subset.py
index a621749..b27b793 100644
--- a/Lib/fontTools/subset.py
+++ b/Lib/fontTools/subset.py
@@ -1508,9 +1508,9 @@
       else:
         all_privs.append(font.Private)
       for priv in all_privs:
-        priv.BlueValues = []
-        for k in ['OtherBlues', 'StemSnapH', 'StemSnapV', 'StdHW', 'StdVW', \
-                  'FamilyBlues', 'FamilyOtherBlues']:
+        for k in ['BlueValues', 'OtherBlues', 'FamilyBlues', 'FamilyOtherBlues',
+                  'BlueScale', 'BlueShift', 'BlueFuzz',
+                  'StemSnapH', 'StemSnapV', 'StdHW', 'StdVW']:
           if hasattr(priv, k):
             setattr(priv, k, None)