Handle undefined and null in form-data field value

[freaz]

Description

Fields with value null or undefined are not sent, as they are not convertible to string or Blob.

Motivation and Context

Internally reported bug.

Types of changes

• 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 change)

Checklist:

• I have updated the documentation accordingly. If you are changing code related to user secrets you need to really make sure that security documentation is correct.
• I have read the CONTRIBUTING document.
• I haven't repeated the code. (DRY)
• I have added tests to cover my changes.
• All new and existing tests passed.