title: R dplyr method to replace all empty factors with NA
body: <p>Instead of writing and reading a dataframe to fill all empty factors in this method, </p>
  
  <pre><code>na.strings=c("","NA")
  </code></pre>
  
  <p>I wanted to just apply a function to all the columns and substitute the empties with NA. I've selected the factor columns so far but don't know what to do next. </p>
  
  <pre><code>df %&gt;% select_if(is.factor) %&gt;% ....
  </code></pre>
  
  <p>How would I be able to do this, preferably with dplyr and/or apply methods </p>