publishable/database/migrations/2017_04_11_000000_alter_post_nullable_fields_table.php
public function up()
    {
        $platform = \DB::getDoctrineSchemaManager()->getDatabasePlatform();
        $platform->registerDoctrineTypeMapping('enum', 'string');

        Schema::table('posts', function (Blueprint $table) {
            $table->text('excerpt')->nullable()->change();
            $table->text('meta_description')->nullable()->change();
            $table->text('meta_keywords')->nullable()->change();
        });
    }
public function up()
    {
        $platform = \DB::getDoctrineSchemaManager()->getDatabasePlatform();
        $platform->registerDoctrineTypeMapping('enum', 'string');

        Schema::table('posts', function (Blueprint $table) {
            $table->text('excerpt')->nullable()->change();
            $table->text('meta_description')->nullable()->change();
            $table->text('meta_keywords')->nullable()->change();
        });
    }
Run the migrations.

@return void