Insert NULL instead of blank if form fields left empty

<?php
if ($_SERVER['REQUEST_METHOD'] == "POST") {
require('dbconnect.php');
if (isset($_POST['submit'])) {
if (empty($_POST['slist']) || empty($_POST['catelist'])) {
    $showerror = "Location or Category can not be empty";
  }
else{
  $location = $_POST['slist'];
  $bio = (isset($_POST['setbio']) && !empty($_POST['setbio'])) ? $_POST['setbio'] : null;
  $category = $_POST['catelist'];
  $skill1 = (isset($_POST['skill1']) && !empty($_POST['skill1'])) ? $_POST['skill1'] : null;
  $skill2 = (isset($_POST['skill2']) && !empty($_POST['skill2'])) ? $_POST['skill2'] : null;
  $skill3 = (isset($_POST['skill3']) && !empty($_POST['skill3'])) ? $_POST['skill3'] : null;
$sql = "INSERT INTO user_info(s_location, s_bio, s_category, s_skill1,s_skill2,s_skill3) VALUES (:location,:bio,:category,:s1,:s2,:s3)";
  $stmt=$conn->prepare($sql);
  $stmt->bindPARAM(':location',$location);
  $stmt->bindPARAM(':bio',$bio);
  $stmt->bindPARAM(':category',$category);
  $stmt->bindPARAM(':s1',$skill1);
  $stmt->bindPARAM(':s2',$skill2);
  $stmt->bindPARAM(':s3',$skill3);
 if ($stmt->execute()) {
  echo"success";
 }
 else{
  echo"some error occoured";
 }
}
}
}
?>

<form class="row g-3" method="POST" action="users.php">
<select name="slist" id="slist" required>
 <option value="">Select State</option>
 <option value="1">state 1</option>
 <option value="2">state 2</option>
 <option value="3">state 3</option>
</select>
<textarea rows="5" name="setbio" id="setbio" maxlength="600"></textarea>
<select name="catelist" id="catelist" required>
  <option value="">Select Category</option>
  <option value="1">Category 1</option>
  <option value="2">Category 2</option>
  <option value="3">Category 3</option>
</select>
<input  type="text" id="skill1" name="skill1" placeholder="1">
<input type="text" id="skill2" name="skill2" placeholder="2">
<input type="text" id="skill3" name="skill3" placeholder="3">
</form>