php编辑配置文件当用户不提供密码时如何检查其是否为空

我有一个表单供用户编辑他们的配置文件：

 <div id="page">
   <?php echo message(); ?>
   <?php echo form_errors($errors); ?>
   <h2>Edit Admin: <?php echo htmlentities($admin["username"]); ?></h2>
   <form action="edit_admin.php?usr_serno=<?php echo urlencode($admin["usr_serno"]); ?>"  method="post">
   <p>Username:
   <input type="text" name="username" value="<?php echo htmlentities($admin["username"]); ?>" />
   </p>
   <p>Password:
   <input type="password" name="password" value="" />
   <?php if (!isset($_GET["password"])
   </p>
   <p>Email:
   <input type="text" name="email" value="<?php echo htmlentities($admin["email"]); ?>" />
   </p>
   <p>Role:
   <select type="int" name="role_serno">
   <option value="1">Administrator</option>
   <option value="2">User</option>
   </select>
   </p>
   <p>First Name:
   <input type="text" name="name" value="<?php echo htmlentities($admin["name"]); ?>" />
   </p>
   <p>Last Name:
   <input type="text" name="lastname" value="<?php echo htmlentities($admin["lastname"]); ?>" />
   </p>
   <p>ID:
   <input type="text" name="id" value="<?php echo htmlentities($admin["id"]); ?>" />
   </p>
   <p>Address:
   <input type="text" name="address" value="<?php echo htmlentities($admin["address"]); ?>" />
  </p>
  <p>Postcode:
    <input type="text" name="postcode" value="<?php echo htmlentities($admin["postcode"]); ?>" />
  </p>
  <p>City:
  <select id="city" type="text" name="city" value="<?php echo htmlentities($admin["city"]); ?>">
   <option value="Nicosia">Nicosia</option>
   <option value="Limassol">Limassol</option>
   <option value="Larnaca">Larnaca</option>
   <option value="Paphos">Paphos</option>
   <option value="Other">Other</option>
   </select>
   <input id="other_city" type="text" name="other_city" value="<?php echo htmlentities($admin["city"]); ?>" />
  </p>      
  <p>Telephone:
    <input type="text" name="telephone" value="<?php echo htmlentities($admin["telephone"]); ?>" />
  </p>
  <p>College:
    <input type="text" name="college" value="<?php echo htmlentities($admin["college"]); ?>" />
  </p>
  <input type="submit" name="submit" value="Edit Admin" />
  </form>
  <br />
  <a href="manage_admins.php">Cancel</a>
  </div>

`

在这个页面的顶部，我正在检查并尝试更新用户档案：

if (isset($_POST["submit"])) {
$required_fields = array("username","email","role_serno",
"name","lastname","id","telephone","address","college","postcode","city");
validate_presences($required_fields);
$fields_with_max_lengths = array("username" => 30);
validate_max_lengths($fields_with_max_lengths);
if (empty($errors)) {
// Perform Update
//if (!isset ($_POST["password"]))
//$hashed_password = $_GET["password"];
//if (isset ($_POST["password"]))
$id_serno = $admin["usr_serno"];
$username = mysql_prep($_POST["username"]);
$hashed_password = password_encrypt($_POST["password"]);
$email=mysql_prep($_POST["email"]);
$role_serno=mysql_prep($_POST["role_serno"]);
$name=mysql_prep($_POST["name"]);
$lastname=mysql_prep($_POST["lastname"]);
$id=mysql_prep( $_POST["id"]);
$date_create=mysql_prep( $_POST["date_create"]);
$address=mysql_prep( $_POST["address"]);
$postcode=mysql_prep( $_POST["postcode"]);
$city=mysql_prep( $_POST["city"]);
$telephone=mysql_prep( $_POST["telephone"]);
$college=mysql_prep( $_POST["college"]);
$query  = "UPDATE users SET ";
$query .= "username = '{$username}', ";
$query .= "password = '{$hashed_password}', ";
$query .= "email = '{$email}', ";
$query .= "role_serno = '{$role_serno}', ";
$query .= "name = '{$name}', ";
$query .= "lastname = '{$lastname}', ";
$query .= "id = '{$id}', ";
$query .= "address = '{$address}', ";
$query .= "postcode = '{$postcode}', ";
$query .= "city = '{$city}', ";
$query .= "telephone = '{$telephone}', ";
$query .= "college = '{$college}' ";
$query .= "WHERE usr_serno = {$id_serno} ";
$query .= "LIMIT 1";
$result = mysqli_query($connection, $query);
if ($result && mysqli_affected_rows($connection) == 1) {
  // Success
  $_SESSION["message"] = "Admin updated.";
  redirect_to("manage_admins.php");
} else {
  // Failure
  $_SESSION["message"] = "Admin update failed.";
}
}
} else {
// This is probably a GET request
} // end: if (is set($_POST["submit"]))`

当用户在没有传递密码的情况下发布表单时，问题就会出现，因为数据库会用空密码更新，我在lo gin页面上检查了不接受空密码。

有没有办法避免和检查这种情况。

请告知，因为我是php开发的新手，我不能100%确定如何做到这一点。

到目前为止，我试图检查post值是否为空，然后在不编码的情况下传递当前数据库值。但由于某种原因，if检查实际上并没有起到任何作用