If error with zero-length reponse, set error message to "Empty Body" …

[gadenbuie]

…so that correct error is thrown. This is common with conditional requests where a 304 error is returned with an empty body to indicate that the resource hasn't been modified.

Currently, gh fails to return the correct error from gh_error() when res is raw(0) (an empty response body).

library(gh)

issue <- "/repos/r-lib/gh/issues/1"
e_first <- gh(issue)
last_modified <- attributes(e_first)$response$`last-modified`

# Conditional request: https://developer.github.com/v3/#conditional-requests
gh(issue, .send_headers = c("If-Modified-Since" = last_modified))
#> Error: $ operator is invalid for atomic vectors

This PR checks if length(res) == 0 and if so initializes res = list(message = "Empty Body").

# With the change from this PR...
gh(issue, .send_headers = c("If-Modified-Since" = last_modified))
#> Error in gh_process_response(raw): 
#> GitHub API error (304): 304 Not Modified
#> Message: Empty Body

I tried to create a test based on the above reprex, but couldn't quickly figure out how to setup httrmock. Hopefully the above reprex is a good start.

[gadenbuie]

Rolled into #117