getBacklinks validation for Constellation

[kotx]

For blue.microcosm.links.getBacklinks, Constellation can return {"total":0,"records":[],"cursor":null}.
When using Client#call(BlueMicrocosmLinksGetBacklinks, ...) to perform validation, it fails due to cursor being null:

validation failed for output: invalid_type at .cursor (expected string)

Example URL: https://constellation.microcosm.blue/xrpc/blue.microcosm.links.getBacklinks?subject=at%3A%2F%2Fdid%3Aplc%3Aofyv3texbybqcblh3ws4snnp%2Fclick.catnip.thread%2F3maf27bp4ec2f&source=click.catnip.thread.comment%3Areply.root.uri

[mary-ext]

ouch wasn't expecting cursor to be null, I'll fix it here and make a PR upstream

[uniphil]

wait constellation is wrong to do nulls i think by the xrpc spec, and should omit instead (existing lexicons are how they should be)

i'm leaning toward breaking change in constellation to match the lexicons. going to sleep on it.