fix ForeignKeyField is none when fk is integer 0 #1274

[liumuqing]

Description

This PR fixes a bug in _fk_getter when fk is 0, similar to #1055

Motivation and Context

How Has This Been Tested?

I test case is added in this PR

Checklist:

• [ x ] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• [ x ] I have read the CONTRIBUTING document.
• [ x ] I have added tests to cover my changes.
• [ x ] All new and existing tests passed.

[long2ice]

Thanks!