feat: avoid replacing unix_fds if there are no unix_fds

[bdraco]

No description provided.

[codecov]

Codecov Report

Base: 82.31% // Head: 82.17% // Decreases project coverage by -0.14% ⚠️

Coverage data is based on head (6ea1296) compared to base (fb4d540).
Patch coverage: 94.11% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   82.31%   82.17%   -0.15%     
==========================================
  Files          27       27              
  Lines        3144     3147       +3     
  Branches      649      650       +1     
==========================================
- Hits         2588     2586       -2     
- Misses        340      343       +3     
- Partials      216      218       +2     

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/_private/util.py	86.79% <100.00%> (ø)
src/dbus_fast/aio/message_bus.py	85.78% <100.00%> (ø)
src/dbus_fast/message_bus.py	71.32% <100.00%> (+0.05%)	⬆️
src/dbus_fast/service.py	88.65% <100.00%> (+0.07%)	⬆️
src/dbus_fast/glib/message_bus.py	80.32% <0.00%> (-2.05%)	⬇️

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