check nillable property for type aliases as well

[Link512]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

• Fixes Using any keyword together with nil causes panic in generated code #820

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Go used when building/testing:

• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20
• 1.23

How Has This Been Tested?

Added a unit test.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[kgadams]

Hi, I can confirm that this code fixes a panic when returning nil from a mock at a point where a type alias is expected.
Please merge this urgently.

[LandonTClipp]

Looks good, thanks!