Make query parameters without = have empty string values

This was reverted in 77cf057
so that Rails could have historical Rack behavior without the
complexity of splitting form/query parsing.

Now that splitting form/query parsing has been added back in
3855d1d, this changes the
behavior to what originally shipped in Rack 3.

This matches URL spec section 5.1.3.3.  Frameworks that want
Rack's historical behavior of using nil values instead of
empty string values can reparse QUERY_STRING and use
rack.request.form_pairs to get the behavior they want.

lib/rack/query_parser.rb

@@ -129,6 +129,8 @@ def normalize_params(params, name, v, _depth=nil)
 
       return if k.empty?
 
+      v ||= String.new
+
       if after == ''
         if k == '[]' && depth != 0
           return [v]

test/spec_query_parser.rb

@@ -12,6 +12,6 @@
   it "can normalize values with missing values" do
     query_parser.parse_nested_query("a=a").must_equal({"a" => "a"})
     query_parser.parse_nested_query("a=").must_equal({"a" => ""})
-    query_parser.parse_nested_query("a").must_equal({"a" => nil})
+    query_parser.parse_nested_query("a").must_equal({"a" => ""})
   end
 end

test/spec_request.rb

@@ -583,9 +583,9 @@ def self.req(headers)
   it "parse the query string" do
     request = make_request(Rack::MockRequest.env_for("/?foo=bar&quux=bla&nothing&empty="))
     request.query_string.must_equal "foo=bar&quux=bla&nothing&empty="
-    request.GET.must_equal "foo" => "bar", "quux" => "bla", "nothing" => nil, "empty" => ""
+    request.GET.must_equal "foo" => "bar", "quux" => "bla", "nothing" => "", "empty" => ""
     request.POST.must_be :empty?
-    request.params.must_equal "foo" => "bar", "quux" => "bla", "nothing" => nil, "empty" => ""
+    request.params.must_equal "foo" => "bar", "quux" => "bla", "nothing" => "", "empty" => ""
   end
 
   it "handles invalid unicode in query string value" do

test/spec_utils.rb

@@ -141,7 +141,7 @@ def assert_nested_query(exp, act)
 
   it "parse nested query strings correctly" do
     Rack::Utils.parse_nested_query("foo").
-      must_equal "foo" => nil
+      must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo=").
       must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo=bar").
@@ -158,7 +158,7 @@ def assert_nested_query(exp, act)
     Rack::Utils.parse_nested_query("&foo=1&&bar=2").
       must_equal "foo" => "1", "bar" => "2"
     Rack::Utils.parse_nested_query("foo&bar=").
-      must_equal "foo" => nil, "bar" => ""
+      must_equal "foo" => "", "bar" => ""
     Rack::Utils.parse_nested_query("foo=bar&baz=").
       must_equal "foo" => "bar", "baz" => ""
     Rack::Utils.parse_nested_query("my+weird+field=q1%212%22%27w%245%267%2Fz8%29%3F").
@@ -168,19 +168,19 @@ def assert_nested_query(exp, act)
       must_equal "pid=1234" => "1023", "a" => "b"
 
     Rack::Utils.parse_nested_query("foo[]").
-      must_equal "foo" => [nil]
+      must_equal "foo" => [""]
     Rack::Utils.parse_nested_query("foo[]=").
       must_equal "foo" => [""]
     Rack::Utils.parse_nested_query("foo[]=bar").
       must_equal "foo" => ["bar"]
     Rack::Utils.parse_nested_query("foo[]=bar&foo").
-      must_equal "foo" => nil
+      must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo[]=bar&foo[").
-      must_equal "foo" => ["bar"], "foo[" => nil
+      must_equal "foo" => ["bar"], "foo[" => ""
     Rack::Utils.parse_nested_query("foo[]=bar&foo[=baz").
       must_equal "foo" => ["bar"], "foo[" => "baz"
     Rack::Utils.parse_nested_query("foo[]=bar&foo[]").
-      must_equal "foo" => ["bar", nil]
+      must_equal "foo" => ["bar", ""]
     Rack::Utils.parse_nested_query("foo[]=bar&foo[]=").
       must_equal "foo" => ["bar", ""]
 
@@ -192,7 +192,7 @@ def assert_nested_query(exp, act)
       must_equal "foo" => ["bar"], "baz" => ["1", "2", "3"]
 
     Rack::Utils.parse_nested_query("x[y][z]").
-      must_equal "x" => { "y" => { "z" => nil } }
+      must_equal "x" => { "y" => { "z" => ""} }
     Rack::Utils.parse_nested_query("x[y][z]=1").
       must_equal "x" => { "y" => { "z" => "1" } }
     Rack::Utils.parse_nested_query("x[y][z][]=1").