Dates and/or times that are not required fail validation when submitted as null

[j00bar]

If there's a property in my schema like:

"date_filter": {
  "type": "string",
  "format": "date",
  "nullable": true
}

... and I leave it blank, what gets submitted is null, which then fails validation for not being a string.

I think you want to change this line to:

            trimmed_data = data or ""

so that null gets converted to an empty string. Either that or moving the if is_empty check above the type check for string.