[Java/Jaxrs/Resteasy] NPE thrown when response is defined with $ref without content

We wanted to fix swagger-api/swagger-codegen/issues/8859 in OpenAPITools/openapi-generator thanks to this PR OpenAPITools/openapi-generator/pull/1416

Then we faced this issue OpenAPITools/openapi-generator/issues/1420.
It seems to come from swagger parser.

Here is the exception :

[WARNING] Exception while reading:
java.lang.NullPointerException
	at io.swagger.v3.parser.ResolverCache.updateLocalRefs(ResolverCache.java:162)
	at io.swagger.v3.parser.ResolverCache.loadRef(ResolverCache.java:152)
	at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalResponse(ExternalRefProcessor.java:205)
	at io.swagger.v3.parser.processors.ResponseProcessor.processReferenceResponse(ResponseProcessor.java:76)
	at io.swagger.v3.parser.processors.ResponseProcessor.processResponse(ResponseProcessor.java:38)
	at io.swagger.v3.parser.processors.OperationProcessor.processOperation(OperationProcessor.java:56)
	at io.swagger.v3.parser.processors.PathsProcessor.processPaths(PathsProcessor.java:83)
	at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:49)
	at io.swagger.v3.parser.OpenAPIV3Parser.readLocation(OpenAPIV3Parser.java:53)
	at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:19)
	at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:552)
	at org.openapitools.codegen.plugin.CodeGenMojo.execute(CodeGenMojo.java:582)

Please what is your point of view about this issue?

We used OpenAPITools/openapi-generator 3.3.3-SNAPSHOT (java/jaxrs/resteasy).

Start with the stack trace at ResolverCache.java:162 and follow the response-reference path through ResolverCache.java, ExternalRefProcessor.java, and ResponseProcessor.java. Reproduce the OpenAPI response that uses $ref without content and verify that resolving it no longer throws a NullPointerException.