[typescript] fix: allow 'additionalProperties: true' at top level (#1…

…6526)

* fix: allow 'additionalProperties: true' at top level

* refactor: use same call as other typescript generators

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptClientCodegen.java

@@ -502,7 +502,7 @@ public String getTypeDeclaration(Schema p) {
 
     @Override
     protected void addAdditionPropertiesToCodeGenModel(CodegenModel codegenModel, Schema schema) {
-        codegenModel.additionalPropertiesType = getTypeDeclaration((Schema) schema.getAdditionalProperties());
+        codegenModel.additionalPropertiesType = getSchemaType(ModelUtils.getAdditionalProperties(schema));
         addImport(codegenModel, codegenModel.additionalPropertiesType);
     }