treewide: use use lib.optionalAttrs instead of 'then {}'

nixos/modules/services/continuous-integration/buildkite-agents.nix

@@ -11,7 +11,7 @@ let
       default = null;
       description = lib.mdDoc description;
       type = types.nullOr types.lines;
-    } // (if example == null then {} else { inherit example; });
+    } // (lib.optionalAttrs (example != null) { inherit example; });
   };
   mkHookOptions = hooks: listToAttrs (map mkHookOption hooks);
 

nixos/modules/services/mail/rspamd.nix

@@ -215,7 +215,7 @@ let
       text = v.extraConfig;
     })
     (filterAttrs (n: v: v.extraConfig != "") cfg.workers))
-    // (if cfg.extraConfig == "" then {} else {
+    // (lib.optionalAttrs (cfg.extraConfig != "") {
       "extra-config.inc".text = cfg.extraConfig;
     });
 in

nixos/modules/services/web-apps/mattermost.nix

@@ -86,8 +86,7 @@ let
   mattermostConf = recursiveUpdate
     mattermostConfWithoutPlugins
     (
-      if mattermostPlugins == null then {}
-      else {
+      lib.optionalAttrs (mattermostPlugins != null) {
         PluginSettings = {
           Enable = true;
         };

pkgs/development/coq-modules/tlc/default.nix

@@ -23,9 +23,7 @@
     maintainers = [ maintainers.vbgl ];
   };
 }).overrideAttrs (x:
-  if lib.versionAtLeast x.version "20210316"
-  then {}
-  else {
+  lib.optionalAttrs (lib.versionOlder x.version "20210316") {
     installFlags = [ "CONTRIB=$(out)/lib/coq/${coq.coq-version}/user-contrib" ];
   }
 )

pkgs/development/lisp-modules-obsolete/quicklisp-to-nix-overrides.nix

@@ -4,8 +4,7 @@ let
   skipBuildPhase = x: {
     overrides = y: ((x.overrides y) // { buildPhase = "true"; });
   };
-  multiOverride = l: x: if l == [] then {} else
-    ((builtins.head l) x) // (multiOverride (builtins.tail l) x);
+  multiOverride = l: x: pkgs.lib.optionalAttrs (l != []) ((builtins.head l) x) // (multiOverride (builtins.tail l) x);
   lispName = (clwrapper.lisp.pname or (builtins.parseDrvName clwrapper.lisp.name).name);
   ifLispIn = l: f: if (pkgs.lib.elem lispName l) then f else (x: {});
   ifLispNotIn = l: f: if ! (pkgs.lib.elem lispName l) then f else (x: {});