add nullable support to perl api parameter (OpenAPITools#1690)

Author: wing328

modules/openapi-generator/src/main/resources/perl/api.mustache

@@ -66,13 +66,15 @@ sub {{operationId}} {
     my ($self, %args) = @_;
 
     {{#allParams}}
+    {{^isNullable}}
     {{#required}}
     # verify the required parameter '{{paramName}}' is set
     unless (exists $args{'{{paramName}}'}) {
       croak("Missing the required parameter '{{paramName}}' when calling {{operationId}}");
     }
 
     {{/required}}
+    {{/isNullable}}
     {{/allParams}}
     # parse inputs
     my $_resource_path = '{{{path}}}';