NPE if the server doesn't provide any HTTP content for an error

[chanseokoh]

A user reported an NPE on the Gitter channel.

Caused by: java.lang.NullPointerException
	at com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:889)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3005)
	at com.google.cloud.tools.jib.json.JsonTemplateMapper.readJson(JsonTemplateMapper.java:118)
	at com.google.cloud.tools.jib.json.JsonTemplateMapper.readJson (JsonTemplateMapper.java:118)
	at com.google.cloud.tools.jib.registry.RegistryEndpointCaller.newRegistryErrorException (RegistryEndpointCaller.java:194)

The NPE is when there was an error communicating with the server. Jib tries to parse the content of the error message (supposed to be a JSON) from the server.

      ErrorResponseTemplate errorResponse =
          JsonTemplateMapper.readJson(responseException.getContent(), ErrorResponseTemplate.class);

I noticed that if we pass a null string, JsonTemplateMapper.readJson() throws NPE with the same stacktrace.

JsonTemplateMapper.readJson((String) null, ErrorResponseTemplate.class);

Turns out responseException.getContent() can return null if there was no content from the server. The reason I think NullAway couldn't catch this is that the return value of getContet() basically comes from a method in the Google HTTP Client library.