X11 error when setting _NET_SYSTEM_TRAY_COLORS

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

[SNIP]

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version: 

Binary i3 version:  4.19.1-non-git © 2009 Michael Stapelberg and contributors

(Getting version from running i3, press ctrl-c to abort…)i3: Could not connect to i3 on socket /run/user/1000/i3/ipc-socket.709: Datei oder Verzeichnis nicht gefunden

Config file

Logfile URL:

- Linux Distribution & Version: Still debian testing

- Are you using a compositor (e.g., xcompmgr or compton): No

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I just ran i3 under xtrace and looked for X11 errors (to test if my new code caused any) and found this:
```
002:<:001f: 72: Request(18): ChangeProperty mode=Replace(0x00) window=0x00000000 property=0x12e("_NET_SYSTEM_TRAY_COLORS") type=0x6("CARDINAL") data=0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff,0x0000ffff;
002:>:001f:Error 3=Window: major=18, minor=0, bad=0x00000000, seq=001f
```
`git grep NET_SYSTEM_TRAY_COLORS` says that this must be i3bar. Seems like it tries to change a property on `XCB_NONE` and the X11 server is not happy about that (I did not check why this error is not printed anywhere...).

Feel free to close this as "not a bug", but to me this feels like a bug or at least a bug-waiting-to-happen (since the code looks like a property was set, but it was not (yet) set).