Compare data in source sheet with target sheet and copy missing rows to target sheet with Google Apps Script

Source sheet has 8 columns and target sheet has 9 columns (note first 8 columns are the same, the 9th column is used to set a url link once it has been mailed out). I don't want the target sheet to be sorted. The url link has to be on the appropriate row it pertains to.

This is the code I am working with so far. It does pull and compare the data, but it keeps adding a blank row at the beginning and after each run. I can't figure out why it does that?

function addNewStudents() {

  let ss = SpreadsheetApp.getActiveSpreadsheet()
  let source = ss.getSheetByName('Accepted Students')

  let sourceValues = source.getRange(2, 1, source.getLastRow(), 8).getValues().filter(String)
  //sourceValues.shift()
  //Logger.log(sourceValues)

  let targetSheet = ss.getSheetByName('Confirmation Letters')
  let targetValues = targetSheet.getRange(2, 1, targetSheet.getLastRow(), 8).getValues().filter(String)

  //console.log(targetValues)

  let diff = targetValues.showDif(sourceValues)


  targetValues = (diff.length && diff) ? targetValues.concat(diff) : targetValues
  // console.log(targetValues)
  if (targetValues === '') {
    var ui = SpreadsheetApp.getUi()
    ui.alert("No new students")

  } else {
    targetSheet.getRange(2, 1, targetValues.length, targetValues[0].length).setValues(targetValues)
  }
  //console.log(targetValues.length)
}

Array.prototype.showDif = function (array) {
  let that = this;
  return array.filter(function (r) {
    return !that.some(function (x) {
      return r.join() === x.join()
    })
  })

}

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