Make serial optional to find USB devices

If no serial is provided, then list all available USB devices (which can
be open and having a serial).

PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>

app/src/usb/usb.c

@@ -53,12 +53,15 @@ accept_device(libusb_device *device, const char *serial,
         return false;
     }
 
+    if (serial) {
+        // Filter by serial
         bool matches = !strcmp(serial, device_serial);
         if (!matches) {
             free(device_serial);
             libusb_close(handle);
             return false;
         }
+    }
 
     out->device = libusb_ref_device(device);
     out->serial = device_serial;
@@ -90,8 +93,6 @@ sc_usb_device_destroy_all(struct sc_usb_device *usb_devices, size_t count) {
 ssize_t
 sc_usb_find_devices(struct sc_usb *usb, const char *serial,
                     struct sc_usb_device *devices, size_t len) {
-    assert(serial);
-
     libusb_device **list;
     ssize_t count = libusb_get_device_list(usb->context, &list);
     if (count < 0) {