nixos/shells: enable to nullify already defined aliases

nixos/modules/programs/bash/bash.nix

@@ -33,7 +33,8 @@ let
   '';
 
   bashAliases = 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
@@ -64,7 +65,7 @@ in
           Set of aliases for bash 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 {