caddyhttp: Enable matching empty query string

Caddyfile syntax: query ""

Or a nil matcher in the JSON should also match an empty query string.

See https://caddy.community/t/v2-match-empty-query/8708?u=matt

modules/caddyhttp/matchers.go

@@ -349,19 +349,21 @@ func (MatchQuery) CaddyModule() caddy.ModuleInfo {
 
 // UnmarshalCaddyfile implements caddyfile.Unmarshaler.
 func (m *MatchQuery) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
-	if *m == nil {
-		*m = make(map[string][]string)
-	}
-
 	for d.Next() {
 		var query string
 		if !d.Args(&query) {
 			return d.ArgErr()
 		}
+		if query == "" {
+			continue
+		}
 		parts := strings.SplitN(query, "=", 2)
 		if len(parts) != 2 {
 			return d.Errf("malformed query matcher token: %s; must be in param=val format", d.Val())
 		}
+		if *m == nil {
+			*m = make(map[string][]string)
+		}
 		url.Values(*m).Set(parts[0], parts[1])
 		if d.NextBlock(0) {
 			return d.Err("malformed query matcher: blocks are not supported")
@@ -372,6 +374,9 @@ func (m *MatchQuery) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 
 // Match returns true if r matches m.
 func (m MatchQuery) Match(r *http.Request) bool {
+	if m == nil {
+		return len(r.URL.Query()) == 0
+	}
 	for param, vals := range m {
 		paramVal, found := r.URL.Query()[param]
 		if found {

modules/caddyhttp/matchers_test.go

@@ -528,6 +528,24 @@ func TestQueryMatcher(t *testing.T) {
 			input:    "/?someparam",
 			expect:   false,
 		},
+		{
+			scenario: "nil matcher value should match empty query",
+			match:    MatchQuery(nil),
+			input:    "/?",
+			expect:   true,
+		},
+		{
+			scenario: "nil matcher value should NOT match a non-empty query",
+			match:    MatchQuery(nil),
+			input:    "/?foo=bar",
+			expect:   false,
+		},
+		{
+			scenario: "non-nil matcher should NOT match an empty query",
+			match:    MatchQuery{"": []string{}},
+			input:    "/?",
+			expect:   false,
+		},
 	} {
 
 		u, _ := url.Parse(tc.input)