title: Insert NA values into dataframe blank cells when importing read.csv/read.xlsx
body: <p>The attached screenshot shows part of a dataframe which I have just imported into R from an excel file. In the cells which are blank, I need to insert 'NA'. How can I insert NA into any cell which is blank (whilst leaving the already populated cells alone)?</p>
  
  <p><img src="https://i.stack.imgur.com/Aq57K.png" alt="enter image description here"></p>