sessionProfile tagged properties with omitempty, so an upstream answer
of "properties": [] (or null, or no key at all) reached Velocity with
no properties key. A Yggdrasil root may legitimately answer that way for
a player without a skin. Velocity 3.5.1's GameProfile deserializer
passes the missing key on as null and ImmutableList.copyOf throws, so
that player hangs at login with nothing logged, even though the same
answer sent straight to Velocity is accepted. Mojang always sends
textures, which is why the premium path and the hardware runs never hit
it.
Drop omitempty and replace a nil slice with an empty one before the
response is written. Removing omitempty alone is not enough: a nil
slice marshals as null, which Velocity rejects the same way.
The new subtest feeds the relay [], null and a missing key and expects
"properties":[] every time. The previous handler fails all three.
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