waybar: remove modules-* from defaults

Some prefer to handle modules-* imperatively for more flexibility, e.g.,
via waybar "includes" (https://github.com/Alexays/Waybar/wiki/Configuration).
Current HM defaults modules-* to `[]` instead of null for most other
settings, so they get written to ~/.config/waybar/config even though
no explicit configuration has been made.

And they take precedence over the imperative ones because of how
waybar's includes work:

====
Paths to additional configuration files.
Each file can contain a single object with any of the bar configuration options. In case of duplicate options, the first defined value takes precedence, i.e. including file -> first included file -> etc. Nested includes are permitted, but make sure to avoid circular imports.
For a multi-bar config, the include directive affects only current bar configuration object.
====

modules/programs/waybar.nix

@@ -69,8 +69,8 @@ let
         };
 
         modules-left = mkOption {
-          type = listOf str;
-          default = [ ];
+          type = nullOr (listOf str);
+          default = null;
           description = "Modules that will be displayed on the left.";
           example = literalExpression ''
             [ "sway/workspaces" "sway/mode" "wlr/taskbar" ]
@@ -78,8 +78,8 @@ let
         };
 
         modules-center = mkOption {
-          type = listOf str;
-          default = [ ];
+          type = nullOr (listOf str);
+          default = null;
           description = "Modules that will be displayed in the center.";
           example = literalExpression ''
             [ "sway/window" ]
@@ -87,8 +87,8 @@ let
         };
 
         modules-right = mkOption {
-          type = listOf str;
-          default = [ ];
+          type = nullOr (listOf str);
+          default = null;
           description = "Modules that will be displayed on the right.";
           example = literalExpression ''
             [ "mpd" "custom/mymodule#with-css-id" "temperature" ]

tests/modules/programs/waybar/deprecated-modules-option.nix

@@ -31,8 +31,6 @@ with lib;
                   "modules-center": [
                     "test"
                   ],
-                  "modules-left": [],
-                  "modules-right": [],
                   "test": {}
                 }
               ]

tests/modules/programs/waybar/settings-complex-expected.json

@@ -51,7 +51,6 @@
     "modules-left": [
       "sway/mode"
     ],
-    "modules-right": [],
     "output": [
       "!DP-1"
     ],