Keep empty params when building url query string

[scsmithr]

I have flymake-languagetool-load in my markdown-mode hook, and would get the following errors when creating a new empty markdown buffer:

Debugger entered--Lisp error: (error "peculiar error: 400")
  signal(error ("peculiar error: 400"))
  error("peculiar error: 400")
  flymake-languagetool--handle-finished((:error (error http 400)) #<buffer test2.md<sean>> #f(compiled-function (&rest args) #<bytecode 0xcd6ba83ba4f58a7>))
  apply(flymake-languagetool--handle-finished ((:error (error http 400)) #<buffer test2.md<sean>> #f(compiled-function (&rest args) #<bytecode 0xcd6ba83ba4f58a7>)))
  url-http-activate-callback()
  url-http-content-length-after-change-function(81 123 42)
  url-http-wait-for-headers-change-function(1 127 126)
  url-http-generic-filter(#<process localhost> "HTTP/1.1 400 Bad Request\15\nDate: Sat, 13 Aug 2022 15:45:14 GMT\15\nContent-length: 41\15\n\15\nError: Missing 'text' or 'data' parameter")

The cause of this being the params with text set to "" were being encoded as "text&language=en-US". After this change, the params are encoded as "text=&language=en-US" (note the extra =), and languagetool returns a 200.

[tpeacock19]

nice catch, thank you!