Ensure that an empty locale is always treated the same as "en".

This avoids empty output from GetSymbol() or the formatter.

currency.go

@@ -60,7 +60,7 @@ func GetSymbol(currencyCode string, locale Locale) (symbol string, ok bool) {
 	}
 	enLocale := Locale{Language: "en"}
 	enUSLocale := Locale{Language: "en", Territory: "US"}
-	if locale == enLocale || locale == enUSLocale {
+	if locale == enLocale || locale == enUSLocale || locale.IsEmpty() {
 		// The "en"/"en-US" symbol is always first.
 		return symbols[0].symbol, true
 	}
@@ -87,13 +87,14 @@ func GetSymbol(currencyCode string, locale Locale) (symbol string, ok bool) {
 
 // getFormat returns the format for a locale.
 func getFormat(locale Locale) currencyFormat {
-	var format currencyFormat
 	// CLDR considers "en" and "en-US" to be equivalent.
 	// Fall back immediately for better performance
 	enUSLocale := Locale{Language: "en", Territory: "US"}
-	if locale == enUSLocale {
-		locale = Locale{Language: "en"}
+	if locale == enUSLocale || locale.IsEmpty() {
+		return currencyFormats["en"]
 	}
+
+	var format currencyFormat
 	for {
 		localeID := locale.String()
 		if cf, ok := currencyFormats[localeID]; ok {

currency_test.go

@@ -120,7 +120,8 @@ func TestGetSymbol(t *testing.T) {
 		{"USD", currency.NewLocale("en-AU"), "US$", true},
 		{"USD", currency.NewLocale("es"), "US$", true},
 		{"USD", currency.NewLocale("es-ES"), "US$", true},
-		{"USD", currency.NewLocale(""), "", true},
+		// An empty locale should use "en" data.
+		{"USD", currency.NewLocale(""), "$", true},
 	}
 
 	for _, tt := range tests {

formatter_test.go

@@ -43,6 +43,10 @@ func TestFormatter_Format(t *testing.T) {
 		{"1234.00", "CHF", "de-CH", "CHF\u00a01’234.00"},
 		{"1234.00", "CHF", "sr", "1.234,00\u00a0CHF"},
 
+		// An empty locale should be equivalent to "en".
+		{"1234.59", "USD", "", "$1,234.59"},
+		{"-1234.59", "USD", "", "-$1,234.59"},
+
 		// Arabic digits.
 		{"12345678.90", "USD", "ar", "\u200f١٢٬٣٤٥٬٦٧٨٫٩٠\u00a0US$"},
 		// Arabic extended (Persian) digits.