go.text/language: make Region.ISO3 return "ZZZ" in case of non-exising ISO3

code.
The occurence of a non-existing ISO3 code is rather rare to the point it
seems better to just return something.

R=r
CC=golang-codereviews
https://golang.org/cl/55230043

language/lookup.go

@@ -334,10 +334,10 @@ func (r regionID) String() string {
 
 // ISO3 returns the 3-letter ISO code of r.
 // Note that not all regions have a 3-letter ISO code.
-// In such cases this method returns the empty string.
+// In such cases this method returns "ZZZ".
 func (r regionID) ISO3() string {
 	if r < isoRegionOffset {
-		return ""
+		return "ZZZ"
 	}
 	r -= isoRegionOffset
 	reg := regionISO[r<<2:]
@@ -345,7 +345,7 @@ func (r regionID) ISO3() string {
 	case 0:
 		return altRegionISO3[reg[3]:][:3]
 	case ' ':
-		return ""
+		return "ZZZ"
 	}
 	return reg[0:1] + reg[2:4]
 }

language/lookup_test.go

@@ -196,14 +196,14 @@ func TestRegionISO3(t *testing.T) {
 	tests := []struct {
 		from, iso3, to string
 	}{
-		{"  ", "", ""},
-		{"000", "", ""},
+		{"  ", "ZZZ", "ZZ"},
+		{"000", "ZZZ", "ZZ"},
 		{"AA", "AAA", ""},
 		{"CT", "CTE", ""},
 		{"DY", "DHY", ""},
 		{"EU", "QUU", ""},
 		{"HV", "HVO", ""},
-		{"IC", "", ""},
+		{"IC", "ZZZ", "ZZ"},
 		{"JT", "JTN", ""},
 		{"PZ", "PCZ", ""},
 		{"QU", "QUU", "EU"},
@@ -213,7 +213,7 @@ func TestRegionISO3(t *testing.T) {
 		{"TF", "ATF", ""},
 		{"FX", "FXX", ""},
 		{"ZZ", "ZZZ", ""},
-		{"419", "", ""},
+		{"419", "ZZZ", "ZZ"},
 	}
 	for _, tt := range tests {
 		r, _ := getRegionID(b(tt.from))