Set default for the birthday of a person to null.

database/migrations/2018_09_03_090023_create_people_table.php

@@ -22,7 +22,7 @@ class CreatePeopleTable extends Migration
             $table->string('surname')->nullable();
             
             // Date of birth
-            $table->date('date_of_birth')->nullable()->default();
+            $table->date('date_of_birth')->nullable()->default(null);
             
             // Adress
             $table->string('city')->nullable();