title: Redis - how to RPUSH/LPUSH an empty list
body: <p>We'd like to RPUSH/LPUSH a key with an empty list.<br/>
  This is for consistency reasons: when the key is read with LRANGE than whether the list is empty or not, the rest of the code behaves the same.<br/>
  <br/>
  Why the fact that if a key has an empty list it is deleted is a problem? <br/>
  Because we are using Redis as a cache and would like to differentiate the 2 situations:<br/>
   1. A specific key with corresponding values was not cached yet. In which case we want to calculate the values (takes a long time) and cache them. The outcome of the calculation might be an empty list.<br/>
   2. A key with an empty list was already cached. In which case we would like not to perform the calculations and return an empty list. <br/></p>
  
  <p>The following options don't work:<br/>
  <br/>
   1. rpush key     --> no list value results with "wrong number of arguments".<br/>
   2. rpush key []  --> adds a '[]' item<br/></p>
  
  <p>The (ugly) solution we are currently using is storing a one-item-list with an "EMPTY-ITEM" item, and checking that when we read the list.</p>
  
  <p>Any ideas?</p>
  
  <p>Thank you</p>