Set column as empty if no file is selected when submit - php

Question

I am very new to PHP and was trying to do the file uploading. When I try to submit a form without having any files selected, the database will still insert a value to the attachments column.
My table picture:
Is there any way that I can set the attachments column as empty if there is not file selected for submission. Below is my code:
view_group.php
<form class="forms-sample" enctype='multipart/form-data' method="post">
<div class="modal-body">
<div class="form-group">
<textarea id="status_content" name="status_content" rows="30"></textarea>
</div>
<div class="form-group">
<label for="section_label">Add Attachments</label><br>
<input type="text" class="form-control" id="file" disabled placeholder="Select a file"><br>
<button type="button" class="file-upload-browse btn btn-primary" name="button" onclick="document.getElementById('fileName').click()">Select a file</button>
<input type="file" name="attachmentfile" id="fileName" style="display:none">
</div>
</div>
<div class="modal-footer">
<button class="btn btn-outline-secondary btn-fw">Cancel</button>
<input type="submit" class="btn btn-primary" name="postStatus" value="Post" />
</div>
</form>
GroupsController.php
function postStatus($std_id, $group_id){
$status = new ManageGroupsModel();
$status->std_id = $std_id;
$status->group_id = $group_id;
$status->status_content = $_POST['status_content'];
$status->attachments = time() . $_FILES['attachmentfile']['name'];
$status->target_dir = "../../attachments/groupfiles/";
//target file to save in directory
$status->target_file = $status->target_dir . basename($_FILES["attachmentfile"]["name"]);
// select file type
$status->imageFileType = strtolower(pathinfo($status->target_file,PATHINFO_EXTENSION));
// valid file extensions
$status->extensions_arr = array("jpg","jpeg","png","gif","pdf", "doc", "pdf");
if($status->postStatus() > 0) {
$message = "Status posted!";
echo "<script type='text/javascript'>alert('$message');
window.location = '../ManageGroupsView/view_group.php?group_id=".$group_id."'</script>";
}
}
GroupsModel.php
function postStatus() {
$sql = "insert into status(std_id, group_id, status_content, attachments) values(:std_id, :group_id, :status_content, :attachments)";
$args = [':std_id'=> $this->std_id, ':group_id'=> $this->group_id, ':status_content'=> $this->status_content, 'attachments'=> $this->attachments];
move_uploaded_file($_FILES['attachmentfile']['tmp_name'],$this->target_dir.$this->attachments); $stmt = DB::run($sql, $args);
$count = $stmt->rowCount(); return $count; }
I'm sorry if the question sounded dumb. It would be really great if someone can help. Thank you!
UPDATE!
Thanks to the people in my comment section, I finally get to do it. Here are the code needed to add :
function postStatus($std_id, $group_id){
$status = new ManageGroupsModel();
$status->std_id = $std_id;
$status->group_id = $group_id;
$status->status_content = $_POST['status_content'];
if($_FILES['attachmentfile']['name']) {
$status->attachments = time() . $_FILES['attachmentfile']['name'];
$status->target_dir = "../../attachments/groupfiles/";
//target file to save in directory
$status->target_file = $status->target_dir . basename($_FILES["attachmentfile"]["name"]);
// select file type
$status->imageFileType = strtolower(pathinfo($status->target_file,PATHINFO_EXTENSION));
// valid file extensions
$status->extensions_arr = array("jpg","jpeg","png","gif","pdf", "doc", "pdf");
} else {
$status->attachments = '';
}
if($status->postStatus() > 0) {
$message = "Status posted!";
echo "<script type='text/javascript'>alert('$message');
window.location = '../ManageGroupsView/view_group.php?group_id=".$group_id."'</script>";
}
}