Fix TestFileParser to support empty strings

Previously empty strings would be replaced with the token name, e.g. "string" in this case.

test/libsolidity/util/TestFileParser.cpp

@@ -503,9 +503,9 @@ void TestFileParser::Scanner::scanNextToken()
 		return TokenDesc{Token::Identifier, _literal};
 	};
 
-	auto selectToken = [this](Token _token, std::string const& _literal = "") -> TokenDesc {
+	auto selectToken = [this](Token _token, std::optional<std::string> _literal = std::nullopt) -> TokenDesc {
 		advance();
-		return make_pair(_token, !_literal.empty() ? _literal : formatToken(_token));
+		return make_pair(_token, _literal.has_value() ? *_literal : formatToken(_token));
 	};
 
 	TokenDesc token = make_pair(Token::Unknown, "");

test/libsolidity/util/TestFileParser.h

@@ -120,8 +120,6 @@ private:
 		std::string m_line;
 		std::string::const_iterator m_char;
 
-		std::string m_currentLiteral;
-
 		TokenDesc m_currentToken;
 	};