fix: Add omitempty to multipartField

[boitsov14]

Description of the Change

• Add OmitEmpty field to multipartField struct.
• Add "omitempty" tag to FolderID field of CreateRequest struct in drive/files.

Issue or RFC

folderId in drive/files/create is optional. related to #95

Alternate Designs

none

Possible Drawbacks

none

Verification Process

I uploaded both images with and without folder-id set. If I set folder-id, then an image was uploaded to the specified folder. If I set folder-id empty, then an image was uploaded to the root folder.

Release Notes

• Add omitempty to multipartField

[coveralls]

coverage: 83.628% (-0.04%) from 83.665% when pulling 46fd9cf7c97d35533e5346f2f5e12a0f491e6c6a on boitsov14:add-omitempty-multipartfield into 51b8fd65a48fe67100c4bbe5f64798392a0ff572 on yitsushi:main.

[yitsushi]

Thank you. Please fix the lint issue and it's ready to merge

[boitsov14]

Thank you for reviewing! I fixed it.