nixos/shells: enable to nullify already defined aliases

nixos/modules/programs/zsh/zsh.nix

@@ -11,7 +11,8 @@ let
   cfg = config.programs.zsh;
 
   zshAliases = concatStringsSep "\n" (
-    mapAttrsFlatten (k: v: "alias ${k}=${escapeShellArg v}") cfg.shellAliases
+    mapAttrsFlatten (k: v: "alias ${k}=${escapeShellArg v}")
+      (filterAttrs (k: v: !isNull v) cfg.shellAliases)
   );
 
 in
@@ -39,7 +40,7 @@ in
           Set of aliases for zsh shell, which overrides <option>environment.shellAliases</option>.
           See <option>environment.shellAliases</option> for an option format description.
         '';
-        type = with types; attrsOf (either str path);
+        type = with types; attrsOf (nullOr (either str path));
       };
 
       shellInit = mkOption {