[BUG] 5.0.0 breaks generator if servers url relative

Created by: UnleashSpirit

Hi recently switch from openapi-generator-maven-plugin@4.3.1 to 5.0.0 and the generation of my swagger did not work anymore.

After hours of search, I finaly found that it was mu servers > url value that breaked the generation. Of course my swagger is valid according to https://apitools.dev/swagger-parser/online/ (and many others). And also specification : https://swagger.io/specification/#server-object

Field Name	Type	Description
url	string	REQUIRED. A URL to the target host. This URL supports Server Variables and MAY be relative

Step to reproduce :

Take the classic petstore : https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml Change

servers:
  - url: http://petstore.swagger.io/v1

servers:
  - url: /v1

Run

java -jar openapi-generator-cli.jar generate -i <path_to/petstore.yaml> -g spring

Breaks with error (unclear at all) com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'openapi': was expecting (JSON String, Number, Array, Object or token 'null', 'true' or 'false')

I guess it breaks all generator cause it also break validate command.