Previously, XhrBackend would call JSON.parse('') if the response body was
empty (a 200 status code with content-length 0). This changes the XhrBackend
to attempt the JSON parse only if the response body is non-empty. Otherwise,
the body is left as null.
Fixes#18680.
Fixes#19413.
Fixes#19502.
Fixes#19555.
PR Close#19958