facebook: use default empty string for primary keys instead of required

[mdibaiee]

Description:

• Use a default string so we can capture these collections even though sometimes the keys do not exist in certain documents. In these cases those documents will get reduced together (this is what has happened previously as well, since these fields were not even marked as a key)

Workflow steps:

(How does one use this feature, and how has it changed)

Documentation links affected:

(list any documentation links that you created, or existing ones that you've identified as needing updates, along with a brief description)

Notes for reviewers:

(anything that might help someone review this PR)

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This change is 

[jgraettinger]

This is potentially applied to format: date fields. Default values must pass all schema validations. If you tried to actually preview a collection with these schemas I believe they'd failed with a build error. The default would need to be aware of this and use a valid date or date-time sentinel (depending on format).