javascript: how can I print an error message but only if there is already something in the text box

Question

I have 2 text boxes. Inputs cannot match but I don't want error message to show up when they're both empty only when there has been something inputed in both boxes. how do I do this?

function validate() {
  var errorMessage = "";

  var textbox4Input = document.getElementById("fictional");
  var textbox5Input = document.getElementById("real");



  var textbox4 = textbox4Input.value.trim();
  var textbox5 = textbox5Input.value.trim();



  textbox4Input.value = textbox4;
  textbox5Input.value = textbox5;

if (textbox4 === textbox5) {
    errorMessage += "Favorite fictional place & favorite real place cannot be the same.<br>"
}
  return errorMessage;

}