hclwrite: Allow blank quoted string block labels

The hclsyntax package permits block labels to be blank quoted strings,
and defers to the application to determine if this is valid or not. This
commit updates hclwrite to allow the same behaviour.

This is in response to an upstream bug report on Terraform, which uses
hclwrite to implement its fmt subcommand. Given a block with a blank
quoted string label, it currently deletes the label when formatting.
This is inconsistent with Terraform's behaviour when parsing HCL, which
treats empty labels differently from missing labels.

hclwrite/ast_block.go

@@ -159,6 +159,12 @@ func (bl *blockLabels) Current() []string {
 				if !diags.HasErrors() {
 					labelNames = append(labelNames, labelString)
 				}
+			} else if len(tokens) == 2 &&
+				tokens[0].Type == hclsyntax.TokenOQuote &&
+				tokens[1].Type == hclsyntax.TokenCQuote {
+				// An open quote followed immediately by a closing quote is a
+				// valid but unusual blank string label.
+				labelNames = append(labelNames, "")
 			}
 
 		default:

hclwrite/ast_block_test.go

@@ -107,6 +107,13 @@ escape "\u0041" {
 `,
 			[]string{"\u0041"},
 		},
+		{
+			`
+blank "" {
+}
+`,
+			[]string{""},
+		},
 	}
 
 	for _, test := range tests {
@@ -414,7 +421,7 @@ func TestBlockSetLabels(t *testing.T) {
 		{
 			`foo "hoge" /* foo */  "" {}`,
 			"foo",
-			[]string{"hoge"},
+			[]string{"hoge", ""},
 			[]string{"fuga"}, // force quoted form even if the old one is unquoted.
 			Tokens{
 				{