Fix factory generation for morphs column type

[diogogomeswww]

Right now if we use a morphs column type like:

foo: morphs

We get a factory like this:

This PR will add those 2 fields (id and type) for morphs.
For a column definition like morphs nullable it will not generate factory fields for those 2 columns.

An isNullable() helper method for the Column Model.

Questions:

• Should we add a factory definition for a nullable column?
  In my opinion we shouldn't. If a column is nullable, the factory should not have a definition for that column, otherwise we don't have a way to set it to null. If we send a null value for a key, the factory will generate the value for that key.

• Should we add support for nullableMorphs, nullableUuidMorphs, nullableTimestamps column definitions that are availabe on Laravel.

So right now this works:

foo: morphs nullable

But this doesn't

foo: nullableMorphs

[jasonmccreary]

Thanks for this. Two quick things before merging...

1. Can you rename nullable column to bar in your test fixture. Was just a bit confusion.
2. Now that you have added the isNullable method, can you refactor other areas of the code to use this as well.

[diogogomeswww]

Requested changes done.