Support nullable in typescript-fetch client (#3645)

Author: aiham

modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache

@@ -17,7 +17,7 @@ import {
 {{#allParams.0}}
 export interface {{operationIdCamelCase}}Request {
     {{#allParams}}
-    {{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}};
+    {{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{#isNullable}} | null{{/isNullable}}{{/isEnum}};
     {{/allParams}}
 }
 
@@ -238,7 +238,7 @@ export class {{classname}} extends runtime.BaseAPI {
     {{/summary}}
     */
     {{^useSingleRequestParameter}}
-    async {{nickname}}({{#allParams}}{{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{#hasMore}}, {{/hasMore}}{{/allParams}}): Promise<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}}> {
+    async {{nickname}}({{#allParams}}{{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{#isNullable}} | null{{/isNullable}}{{/isEnum}}{{#hasMore}}, {{/hasMore}}{{/allParams}}): Promise<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}}> {
         {{#returnType}}
         const response = await this.{{nickname}}Raw({{#allParams.0}}{ {{#allParams}}{{paramName}}: {{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}} }{{/allParams.0}});
         return await response.value();

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -24,7 +24,7 @@ export interface {{classname}} {{#parent}}extends {{{parent}}} {{/parent}}{
      * @type {{=<% %>=}}{<%&datatype%>}<%={{ }}=%>
      * @memberof {{classname}}
      */
-    {{#isReadOnly}}readonly {{/isReadOnly}}{{name}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{datatype}}}{{/isEnum}};
+    {{#isReadOnly}}readonly {{/isReadOnly}}{{name}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{datatype}}}{{#isNullable}} | null{{/isNullable}}{{/isEnum}};
 {{/vars}}
 }