services: replace bare serializers with (serializer ...)

* gnu/home/services/shells.scm
(home-zsh-configuration)[environment-variables]: Use (serializer ...).
(home-bash-configuration)[aliases, environment-variables]: Likewise.
(home-fish-configuration)[abbreviations, aliases]
[environment-variables]: Likewise.
* gnu/services/audio.scm (mpd-configuration)[music-dir, playlist-dir]
[endpoints, address, inputs, archive-plugins, input-cache-size]
[decoders, filters, playlist-plugins]: Likewise.
* gnu/services/linux.scm (fstrim-configuration)[extra-arguments]:
Likewise.
* gnu/services/security.scm (fail2ban-jail-configuration)[backend]
[log-encoding, extra-content]: Likewise.
* tests/services/configuration.scm: Update tests.
("serialize-configuration [deprecated]"): New test.

Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

gnu/services/linux.scm

@@ -213,9 +213,10 @@ option in fstab are skipped.")
    maybe-list-of-strings
    "Extra options to append to @command{fstrim} (run @samp{man fstrim} for
 more information)."
-   (lambda (_ value)
-     (if (maybe-value-set? value)
-         value '())))
+   (serializer
+    (lambda (_ value)
+      (if (maybe-value-set? value)
+          value '()))))
   (prefix fstrim-))
 
 (define (serialize-fstrim-configuration config)