[5.3] Fix storedAs virtualAs

[fernandobandeira]

Fixes #16643

Currently the Grammar is appending not null and null when generating the virtualAs and storedAs columns.

This is an invalid syntax since these are generated columns so you can't use that, currently it's throwing:

Syntax error or access violation: 1064 You have an error in your SQL syntax

Example of current sql:

alter table `products` add `price` int not null, add `discounted_virtual` int as (price - 5) not null, add `discounted_stored` int as (price - 5) not null stored

after this fix

alter table `products` add `price` int not null, add `discounted_virtual` int as (price - 5), add `discounted_stored` int as (price - 5) stored

[chuck-wood]

It would be nice to mention this in the documentation. It's just lucky that I happened to notice that my generated columns were lacking their constraints before deploying to production! I am running MySQL, which does support this feature.

I'm on Laravel 5.5, not sure if this has changed in later versions. Thanks!