Fix parsing of queries with just "?" in URI.parse/1 (#7565)

Closes #7563

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: whatyouhide

lib/elixir/lib/uri.ex

@@ -422,11 +422,13 @@ defmodule URI do
 
     parts = Regex.run(regex, string)
 
-    destructure [_, _, scheme, _, authority, path, _, query, _, fragment], parts
+    destructure [_, _, scheme, _, authority, path, query_with_question_mark, _, _, fragment],
+                parts
+
     scheme = nillify(scheme)
     authority = nillify(authority)
     path = nillify(path)
-    query = nillify(query)
+    query = nillify_query(query_with_question_mark)
     {userinfo, host, port} = split_authority(authority)
 
     scheme = scheme && String.downcase(scheme)
@@ -444,6 +446,10 @@ defmodule URI do
     }
   end
 
+  defp nillify_query("?"), do: ""
+  defp nillify_query("?" <> query), do: query
+  defp nillify_query(_other), do: nil
+
   # Split an authority into its userinfo, host and port parts.
   defp split_authority(string) do
     regex = Regex.recompile!(~r/(^(.*)@)?(\[[a-zA-Z0-9:.]*\]|[^:]*)(:(\d*))?/)

lib/elixir/test/elixir/uri_test.exs

@@ -306,6 +306,10 @@ defmodule URITest do
       assert URI.parse("http://example.com/#").fragment == ""
       assert URI.parse("http://example.com/test#").fragment == ""
     end
+
+    test "preserves an empty query" do
+      assert URI.parse("http://foo.com/?").query == ""
+    end
   end
 
   test "default_port/1,2" do