Don't form empty queries; closes #451

[jennybc]

See #451

Before:

library(httr)
modify_url("http://google.com", query = list(a = NULL))
#> [1] "http://google.com/?="
modify_url("http://google.com", query = list())
#> [1] "http://google.com/?"

After:

devtools::load_all()
#> Loading httr
modify_url("http://google.com", query = list(a = NULL))
#> [1] "http://google.com/"
modify_url("http://google.com", query = list())
#> [1] "http://google.com/"

[hadley]

Ooops, I merged slightly too soon. Would you mind doing another PR to add a bullet to news?