cara Buat Validasi Gambar jika gambar tidak ada namun tetap menyiman gambar defaultnya

 function simpan_realisasi()
	{
		$no_skpd = $this->input->post("no_skpd");
		$jenis_pajak = $this->input->post("jenis_pajak");
		$cicilan = $this->input->post("cicilan");
		$jumlah_pembayaran = $this->input->post("jumlah_pembayaran");
		$nomor_bukti = $this->input->post("nomor_bukti");
		$foto_bukti = $this->input->post("foto_bukti");
		$tanggal_setoran = $this->input->post("tanggal_setoran");

		/*********************************upload***************************/
		$config['upload_path']          = './assets/uploads/';//alamat folder foto yang di upload
		$config['allowed_types']        = 'gif|jpg|jpeg|png|JPEG|PNG|JPG';//pilih ekstensi file yg dibolehkan
		$ext=pathinfo($_FILES['foto_bukti']['name'],PATHINFO_EXTENSION);//Dapatkan extensi file

		$nama=$config['file_name']=time();

		$nama_file=$nama.".$ext";

		$this->load->library('upload', $config);

		if ( ! $this->upload->do_upload('foto_bukti'))
		{
			var_dump($this->upload->display_errors());
		}
		else
		{
			$id_skpd = $this->M_realisasi->cek_no_skpd($no_skpd,$jenis_pajak);
			if($id_skpd>0)
			{
				$data = array
						(
							"id_skpd"=>$id_skpd,
							"jenis_pajak"=>$jenis_pajak,
							"cicilan"=>$cicilan,
							"jumlah_pembayaran"=>$jumlah_pembayaran,
							"nomor_bukti"=>$nomor_bukti,
							"foto_bukti"=>$nama_file,
							"tanggal_setoran"=>$tanggal_setoran
						);
				//print_r($data);
				$this->M_realisasi->realisasi_simpan($data);
				  $this->load->model('M_log');
						$nama=$this->session->userdata('nama');
						$tanggal=date('Y-m-d h:m:s');
						$kegiatan="Input Realisasi
						id_skpd :".$id_realisasi;
						$ip=getenv("REMOTE_ADDR");

						$log=array(
							'nama'=>$nama,
							'tanggal'=>$tanggal,
							'kegiatan'=>$kegiatan,
							'ip'=>$ip
						);
						$this->M_log->simpan_aktifitas($log,'log');
		// Akhir Log
				echo "OK";
			}
		}
		/*********************************upload***************************/
	}