[typescript-angular] Angular 7: An expression of type 'void' cannot be tested for truthiness

[smartym-pb]

formParam.append returns void
but below it is flollowed by logical OR
Here is example of generated code:
let formParams: { append(param: string, value: any): void; };

if (file !== undefined) {
    formParams = formParams.append('file', <any>file) || formParams;
}