This is useful for interpreting relative urls with url_relative().
Examples
catalog <- read_xml(xml2_example("cd_catalog.xml"))
xml_url(catalog)
#> [1] "/tmp/Rtmpa2B6hy/temp_libpathd80a9618c49e2/xml2/extdata/cd_catalog.xml"
x <- read_xml("<foo/>")
xml_url(x)
#> [1] NA