title: R paste: ignore sep if argument is an empty string
body: <p>I'm trying to <code>paste</code> together lines of addresses to a single address string. However, some lines are blank (<code>""</code>) and this means that an extra <code>sep</code> gets inserted and makes the address look ugly. For example:</p>
  
  <pre><code>addr.df &lt;- data.frame(street1=c("22B","Windsor Castle"),street2=c("Baker Street",""),city=c("London","Windsor"))
  
  with(addr.df,paste(street1,street2,city,sep=", "))
  [1] "22B, Baker Street, London" "Windsor Castle, , Windsor"
  </code></pre>
  
  <p>Notice the extra <code>,</code> in the second address. Is there a way round this that doesn't involve a period of regexp induced tourettes?</p>