backend/drm: make serial optional

Merged Simon Ser requested to merge emersion/wlroots:edid-optional-serial into master

The EDID 1.4 spec says that the serial number is optional:

If this field is not used, then enter “00h, 00h, 00h, 00h”.

Leave the wlr_output.serial field NULL in that case, and hide it from the output description.

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