Blank/null IATA code isn't unique when added #679 (#681)

app/Database/migrations/2020_03_28_174238_airline_remove_nullable.php

@@ -0,0 +1,19 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class AirlineRemoveNullable extends Migration
+{
+    public function up()
+    {
+        Schema::table('airlines', function (Blueprint $table) {
+            $table->dropUnique('airlines_iata_unique');
+        });
+    }
+
+    public function down()
+    {
+    }
+}