title: is.na() or any alternative does not pick up NA value
body: <p>I have a data set (mydata) where the last row has a missing first value (the class of the column where the value is missing is "character").</p>
  
  <p>Column names:</p>
  
  <pre><code>A, B, C, D
  </code></pre>
  
  <p>Values:</p>
  
  <pre><code>AAA,10,5,2
  BGF,4,7,10
  BGF,5,13,8
  ,2,3,45
  </code></pre>
  
  <p>While preparing the data I used several functions to check NAs:</p>
  
  <pre><code>anyNA(mydata)
  any(is.na(mydata))
  
  anyNA(mydata$A)
  any(is.na(mydata$A))
  
  any(mydata[,1]=="NA")
  </code></pre>
  
  <p>None of them showed "TRUE". Can you please help me to explain that? Doesn't <code>is.na</code> work with character variables?</p>
  
  <p>P.S. I know there's a thread with a similar title, but the explanation proposed there (my last line with "==") doesn't work.</p>