[typescript-fetch] fix API signature return type for empty 20x responses (OpenAPITools#20249)

* typescript-fetch: fix promise return type for optional responses

* Remove unnecessary change

Author: danowensdev

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

@@ -70,10 +70,10 @@ export interface {{classname}}Interface {
      {{/isDeprecated}}
      */
     {{^useSingleRequestParameter}}
-    {{nickname}}({{#allParams}}{{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{#isNullable}} | null{{/isNullable}}{{/isEnum}}, {{/allParams}}initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<{{{returnType}}}{{^returnType}}void{{/returnType}}>;
+    {{nickname}}({{#allParams}}{{paramName}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{#isNullable}} | null{{/isNullable}}{{/isEnum}}, {{/allParams}}initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<{{{returnType}}}{{#returnType}}{{#isResponseOptional}} | null | undefined {{/isResponseOptional}}{{/returnType}}{{^returnType}}void{{/returnType}}>;
     {{/useSingleRequestParameter}}
     {{#useSingleRequestParameter}}
-    {{nickname}}({{#allParams.0}}requestParameters: {{#prefixParameterInterfaces}}{{classname}}{{/prefixParameterInterfaces}}{{operationIdCamelCase}}Request, {{/allParams.0}}initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<{{{returnType}}}{{^returnType}}void{{/returnType}}>;
+    {{nickname}}({{#allParams.0}}requestParameters: {{#prefixParameterInterfaces}}{{classname}}{{/prefixParameterInterfaces}}{{operationIdCamelCase}}Request, {{/allParams.0}}initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<{{{returnType}}}{{#returnType}}{{#isResponseOptional}} | null | undefined {{/isResponseOptional}}{{/returnType}}{{^returnType}}void{{/returnType}}>;
     {{/useSingleRequestParameter}}
 
 {{/operation}}