go/client/zurl: Drop emptry query parameters for py compatibility

During our work on 'wendelin.core' URI normalization, Kirill Smelkov
noted a inconsistency between NEO/py and NEO/go URI parser [1]: NEO/py
drops empty query options, while NEO/go preserves them. Let's fix this
inconsistency by adjusting NEO/go to NEO/py behaviour.

[1] nexedi/wendelin.core!28 (comment 212447)

go/neo/client.go

@@ -503,6 +503,20 @@ func parseURL(ctx context.Context, u *url.URL) (urlinfo *urlInfo, err error) {
 		return nil, err
 	}
 
+	// py/compatibility: urlib.parse.parse_qs drops empty strings
+	// with default value for 'keep_blank_values' argument [1]. As
+	// we use this (with 'keep_blank_values=False') in NEO/py URI
+	// parser [2], we should also drop empty string query parameters in
+	// NEO/go to ensure the same behaviour between both implementations.
+	//
+	// [1] https://github.com/python/cpython/blob/3.12/Lib/urllib/parse.py#L703-L708
+	// [2] https://lab.nexedi.com/nexedi/neoppod/-/blob/3d435f55/neo/client/zodburi.py#L76
+	for k, v := range q {
+		if v == "" {
+			delete(q, k)
+		}
+	}
+
 	// qpop pops k from query, defaulting to $NEO_<K> if envok.
 	qpop := func(k string, envok bool) string {
 		v, ok := q[k]

go/neo/client_test.go

@@ -699,6 +699,10 @@ func TestParseURL(t *testing.T) {
 	// With query parameters
 	u = "neo://test@127.0.0.1/compress=true&logfile=n.log&cache-size=256"
 	testParseURL(t, u, urlInfo{})
+	// Emtpy query parameters must be ignored (if they wouldn't
+	// be ignored, parseURL should raise an error).
+	u = "neo://test@127.0.0.1?undefined-argument=&another-undefined-argument="
+	testParseURL(t, u, urlInfo{})
 }
 
 // testParseURL tests one zurl for correctness by comparing its parsed