make:entity to generate non-nullable fields

[valx76]

Hey there!

Thinking about the issue #1175.

How about an option in the make:entity command to define the nullable behavior in PHP ?
Some examples:

• Everything (mostly) nullable like the current behavior
• Set the types in PHP as they are specified by the user (and warns that accessing an unassigned non-nullable value will result in errors)
• Same as above but also defines a default value when not nullable (0, empty string, etc.) to avoid unassigned errors

What do you guys think?