String parameter with format date or date-time should fallback to current date if no example or default is defined.

Content & configuration

Example Swagger/OpenAPI definition:

openapi: 3.0.0
info:
  version: "{version}"
  title: String parameter with format date or date-time does not render properly
paths:
  /test:
    get:
      parameters: 
        - in: query
          name: afterDate
          schema:
            type: string
            format: date
        - in: query
          name: afterDateTime
          schema:
            type: string
            format: date-time
      responses:
        "200":
          description: ""

Describe the bug you're encountering

Should use current date for example value. https://github.com/swagger-api/swagger-ui/issues/5061#issuecomment-445286984

To reproduce...

Create string schema with format date or date-time and use it as parameter schema.
Steps to reproduce the behavior:

1. Go to https://editor.swagger.io
2. Paste yaml
3. See placeholder not current date,

Expected behavior

Renders current date as default fallback.

Reproduce the issue in https://editor.swagger.io using the provided OpenAPI YAML, focusing on query parameters whose string schemas use the date and date-time formats without an example or default. Trace the parameter rendering path and verify that the displayed fallback is the current date or date-time rather than an empty or incorrect placeholder.