fix(enumerator): treat an empty sysfs attribute as empty, not an error

[ultramcu]

On Linux, enumerator.readLine (enumerator/usb_linux.go) returns io.EOF when a USB sysfs attribute file exists but is empty (zero bytes). bufio.Reader.ReadLine() returns ("", io.EOF) for an empty file, and readLine passes that EOF through.

parseUSBSysFS reads idVendor, idProduct, serial, manufacturer and product as required fields and propagates the error, so a single USB device that exposes one of these attributes but leaves it blank (some devices ship with an empty serial/manufacturer/product string descriptor) makes the entire GetDetailedPortsList() call fail — no ports are returned at all.

An absent attribute is already handled gracefully (os.IsNotExist → "", nil); an empty one was not.

Fix

Return ("", nil) when ReadLine reports io.EOF, treating an empty attribute the same as a missing one.

line, _, err := reader.ReadLine()
if err == io.EOF {
    return "", nil
}
return string(line), err

Tests

Added enumerator/usb_linux_test.go — all hardware-free (temp files, no real device/sysfs):

• TestReadLineEmptyFile — empty file → "", not an error.
• TestParseUSBSysFSEmptySerial — a device dir with a present-but-empty serial/manufacturer parses successfully (empty SerialNumber).
• TestReadLineMissingFile / TestReadLineContent — regression guards (absent file, normal content with/without trailing newline).

Both new tests fail before the change (return/propagate io.EOF) and pass after. go test -race ./... stays green (verified on Linux).

[cmaglie]

Thank you!