InvokeHttp hits NPE if target system doesn't respond with proper HTTPS Headers

In testing the 0.5.0 RC I found a small bug in InvokeHttp that occurs when the target System doesn't return proper headers when running HTTPS. This line[1] is where it occurs. Without doing too much digging, I believe it occurs because the target system isn't filling out a peer principal or name of the peer principal.

Instead of NPE it should do proper checking for the values before attempting to use them.

[1] https://github.com/apache/nifi/blob/0db2dc9fc3466051869bbf7ad8b2961405554d80/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/InvokeHTTP.java#L941-L941