On macOS it seems that sometimes an event with Ident=0 is delivered, and
no other flags/information beyond that, even though we never saw such a
file descriptor. For example in TestWatchSymlink/277 (usually at the
end, but sometimes sooner):

	# Okay, expected
	CREATE               "/foo"
	REMOVE               "/foo"
	CREATE               "/apple"
	CREATE               "/pear"
	RENAME               "/apple"
	REMOVE               "/pear"

	# One or more write events without any ident or information?!
	WRITE                ""

Printing some debug in the loop right after we receive events shows:

	unix.Kevent_t{Ident:0x2a, Filter:-4, Flags:0x25, Fflags:0x2, Data:0, Udata:(*uint8)(nil)}
	unix.Kevent_t{Ident:0x0,  Filter:-4, Flags:0x25, Fflags:0x2, Data:0, Udata:(*uint8)(nil)}

The first is a normal event, the second with Ident 0. No error flag, no
data, no ... nothing.

I read a bit through bsd/kern_event.c from the xnu source, but I don't
really see an obvious location where this is triggered – this doesn't
seem intentional, but idk...

Technically fd 0 is a valid descriptor, so only skip it if there's no
path, and if we're on macOS.