HttpClientBuilder addExecInterceptorFirst and addExecInterceptorLast NPE

Calling HttpClientBuilder.addExecInterceptorFirst or HttpClientBuilder.addExecInterceptorLast without first using addExecInterceptorBefore, addExecInterceptorBefore, or replaceExecInterceptor results in a NullPointerException.

Unlike other builder methods addExecInterceptorFirst and addExecInterceptorLast do not first do a null check to conditionally initialize the interceptors list.