private static String skipEmptyLines(List<String> lines) {
        StringBuilder sb = new StringBuilder();
        for (String line : lines) {
            // Remove empty lines
            line = line.trim();
            if (line.length() > 0) {
                sb.append(line).append("\n");
            }
        }
        return sb.toString();
    }
Computes xxhash64 of a list of strings. The only change to input lines before
computing xxhash64 is to skip empty lines.

@param lines A list of lines
@return the xxhash64 of the list, with lowercase letters
config/src/test/java/com/yahoo/vespa/config/util/ConfigUtilsTest.java