[Java][Spring] Fix template for reactive implementation with interfaceOnly parameter (#2437)

Author: logarithm

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

@@ -111,11 +111,11 @@ public interface {{classname}} {
         method = RequestMethod.{{httpMethod}})
     {{#jdk8}}default {{/jdk8}}{{#responseWrapper}}{{.}}<{{/responseWrapper}}ResponseEntity<{{>returnTypes}}>{{#responseWrapper}}>{{/responseWrapper}} {{#delegate-method}}_{{/delegate-method}}{{operationId}}({{#allParams}}{{>queryParams}}{{>pathParams}}{{>headerParams}}{{>bodyParams}}{{>formParams}}{{#hasMore}},{{/hasMore}}{{^hasMore}}{{#reactive}}, {{/reactive}}{{/hasMore}}{{/allParams}}{{#reactive}}ServerWebExchange exchange{{/reactive}}){{^jdk8}};{{/jdk8}}{{#jdk8}} {
         {{#delegate-method}}
-        return {{operationId}}({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
+        return {{operationId}}({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}}{{#reactive}}{{#hasParams}}, {{/hasParams}}exchange{{/reactive}});
     }
 
     // Override this method
-    default {{#responseWrapper}}{{.}}<{{/responseWrapper}}ResponseEntity<{{>returnTypes}}>{{#responseWrapper}}>{{/responseWrapper}} {{operationId}}({{#allParams}}{{#isFormParam}}{{#isFile}}MultipartFile{{/isFile}}{{^isFile}}{{{dataType}}}{{/isFile}}{{/isFormParam}}{{^isFormParam}}{{{dataType}}}{{/isFormParam}} {{paramName}}{{#hasMore}},{{/hasMore}}{{^hasMore}}{{#reactive}}, {{/reactive}}{{/hasMore}}{{/allParams}}{{#reactive}}ServerWebExchange exchange{{/reactive}}) {
+    {{#jdk8}}default {{/jdk8}} {{#responseWrapper}}{{.}}<{{/responseWrapper}}ResponseEntity<{{>returnTypes}}>{{#responseWrapper}}>{{/responseWrapper}} {{operationId}}({{#allParams}}{{^isFile}}{{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{{dataType}}}{{/reactive}}{{#reactive}}{{^isListContainer}}Mono{{/isListContainer}}{{#isListContainer}}Flux{{/isListContainer}}<{{{baseType}}}>{{/reactive}}{{/isBodyParam}}{{/isFile}}{{#isFile}}MultipartFile{{/isFile}} {{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}}{{#reactive}}{{#hasParams}}, {{/hasParams}}ServerWebExchange exchange{{/reactive}}) {
         {{/delegate-method}}
         {{^isDelegate}}
         {{>methodBody}}