title: Why isEmpty () not null checked?
body: <p>First, ask for your understanding that English is not fluent.</p>
  
  <p>I am currently implementing an upload function using MultipartFile. It is checking for an empty value with isEmpty() in case there is no file. When i submit without selecting a file, a NullPointerException occurs in the isEmpty () part.</p>
  
  <p>So, when i do not have a file, it works fine if i check <code>file != null</code> instead of isEmpty(). Looking at the implementation of MultipartFile(ex:CommonsMultipartFile, StandardMultipartFile...), this only check if the file size is zero. If so, I wonder if it is OK to check it like <code>file != null</code> to check for null.</p>
  
  <pre><code>if(uploadFile.isEmpty()){ //Here a NullPointerException is thrown
      ...
  }
  
  if(uploadFile != null){ //Does not occur here
      ...
  }
  </code></pre>