easyeffects: handle null extra presets

[khaneliman]

Description

Treat services.easyeffects.extraPresets = null like an empty preset set.
The option type already permits null, but preset generation attempted to map it
as an attribute set.

Keeps paired preset behavior unchanged; #9716 can evolve that separately.

Tested with nix run path:.#tests -- easyeffects.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.

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  or a targeted nix run .#tests -- <pattern>.

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