fix panic when a blockstring has only 1 line and blank

Author: miiton

language/lexer/lexer.go

@@ -427,6 +427,9 @@ func blockStringValue(in string) string {
 
 	// Remove leading blank lines.
 	for {
+		if len(lines) == 1 {
+			break
+		}
 		if isBlank := lineIsBlank(lines[0]); !isBlank {
 			break
 		}

language/lexer/lexer_test.go

@@ -449,6 +449,15 @@ func TestLexer_ReportsUsefulStringErrors(t *testing.T) {
 
 func TestLexer_LexesBlockStrings(t *testing.T) {
 	tests := []Test{
+		{
+			Body: `""""""`,
+			Expected: Token{
+				Kind:  TokenKind[BLOCK_STRING],
+				Start: 0,
+				End:   6,
+				Value: "",
+			},
+		},
 		{
 			Body: `"""simple"""`,
 			Expected: Token{