Inventory dataset form: optional field gives json export error if left blank

[FuhuXia]

When adding a resource in inventory dataset form, if the field "Format", which is marked optiojnal, is left blank, it gives an error during json export.

How to reproduce

• add a resource to a dataset
• choose "Link to a file" as the resource type, leave the format field blank.
• export the datajson for the org.

Expected behavior

json is exported without error

Actual behavior

An error for the dataset during export,

'required': ['mediaType']

Sketch

User reported this issue. #5415 (comment)