HBASE-2037 broke Scan

Paul Ambrose wrote to the mailing list about some tests he has that doesn't pass on 0.20.3RC1-2. Looking into the issue it appears that this modification:

   public Scan addFamily(byte [] family) {
     familyMap.remove(family);
-    familyMap.put(family, null);
+    familyMap.put(family, EMPTY_NAVIGABLE_SET);
     return this;
   }

Makes it that when you use addColumn after that you put qualifiers into EMPTY_NAVIGABLE_SET which is static hence shared among all scanners after that like META scanners when calling tableExists.

This was introduced by HBASE-2037.