Fix default host when parsing URL without host but with port

fixes https://github.com/jackc/pgconn/issues/72

config.go

@@ -411,8 +411,12 @@ func parseURLSettings(connString string) (map[string]string, error) {
 		if err != nil {
 			return nil, fmt.Errorf("failed to split host:port in '%s', err: %w", host, err)
 		}
-		hosts = append(hosts, h)
-		ports = append(ports, p)
+		if h != "" {
+			hosts = append(hosts, h)
+		}
+		if p != "" {
+			ports = append(ports, p)
+		}
 	}
 	if len(hosts) > 0 {
 		settings["host"] = strings.Join(hosts, ",")

config_test.go

@@ -32,6 +32,10 @@ func TestParseConfig(t *testing.T) {
 		}
 	}
 
+	config, err := pgconn.ParseConfig("")
+	require.NoError(t, err)
+	defaultHost := config.Host
+
 	tests := []struct {
 		name       string
 		connString string
@@ -428,6 +432,20 @@ func TestParseConfig(t *testing.T) {
 				},
 			},
 		},
+		// https://github.com/jackc/pgconn/issues/72
+		{
+			name:       "URL without host but with port still uses default host",
+			connString: "postgres://jack:secret@:1/mydb?sslmode=disable",
+			config: &pgconn.Config{
+				User:          "jack",
+				Password:      "secret",
+				Host:          defaultHost,
+				Port:          1,
+				Database:      "mydb",
+				TLSConfig:     nil,
+				RuntimeParams: map[string]string{},
+			},
+		},
 		{
 			name:       "DSN multiple hosts one port",
 			connString: "user=jack password=secret host=foo,bar,baz port=5432 dbname=mydb sslmode=disable",