Only specify column in SQLite schema grammar as NULL if defined as such.

When using an SQLite database, all columns are currently defined as nullable. In other words:

```php
Schema::create('foo', function($table) {
    $table->increments('id');
    $table->integer('bar');
    $table->integer('baz')->nullable();
});
```
Results in:

```sql
create table "foo" ("id" integer null primary key autoincrement, "bar" integer null, "baz" integer null)
```

Instead of the expected:

```sql
create table "foo" ("id" integer not null primary key autoincrement, "bar" integer not null, "baz" integer null)
```

This commit fixes that.