main: keep .conf transport URLs when per-type flags are unset

buildTransportSpecs overrode every [Transport] URL with its per-type flag
(--vyandex-url, --yandex-url, ...) even when the flag was unset, so an
exit started from --config ran vyandex with an empty URL
(Get "": unsupported protocol scheme "") and never reached its document.
Only a flag that was set overrides now.

transport_spec.go

@@ -50,7 +50,9 @@ func parseTransportList(s string) ([]transportSpec, error) {
 // per-type URL flags (--yandex-url, --mailru-url, ...) and global settings.
 func buildTransportSpecs(specs []transportSpec, urls map[string]string, extra map[string]map[string]interface{}) []transportSpec {
 	for i := range specs {
-		if u, ok := urls[specs[i].Type]; ok {
+		// Only a flag that was set overrides: the unset ones are "" and
+		// would wipe the URL of a .conf [Transport] section.
+		if u, ok := urls[specs[i].Type]; ok && u != "" {
 			specs[i].URL = u
 		}
 		if p, ok := extra[specs[i].Type]; ok {

transport_spec_url_test.go

@@ -0,0 +1,21 @@
+package main
+
+import "testing"
+
+// A per-type URL flag overrides a .conf transport's URL only when it is set:
+// the unset flags are "", and overriding with them wiped the document URL of
+// every [Transport] section, so a --config exit started vyandex with no URL.
+func TestBuildTransportSpecsKeepsConfURL(t *testing.T) {
+	specs := []transportSpec{
+		{Name: "vyandex", Type: "vyandex", URL: "https://docs.yandex.ru/edit/d/conf"},
+		{Name: "yandex", Type: "yandex", URL: "https://disk.yandex.ru/i/conf"},
+	}
+	urls := map[string]string{"vyandex": "", "yandex": "https://disk.yandex.ru/i/flag"}
+	got := buildTransportSpecs(specs, urls, nil)
+	if got[0].URL != "https://docs.yandex.ru/edit/d/conf" {
+		t.Fatalf("unset flag wiped the conf URL: %q", got[0].URL)
+	}
+	if got[1].URL != "https://disk.yandex.ru/i/flag" {
+		t.Fatalf("a set flag must override the conf URL: %q", got[1].URL)
+	}
+}