htop: fix htoprc when fields is not explicitly set

When the 'fields' setting is not set in htoprc, the htop program won't read any
of the settings. Provide a default value for fields in case it's not explicitly
set by the user.

tests/modules/programs/htop/default.nix

@@ -1,4 +1,5 @@
 {
   htop-empty-settings = ./empty-settings.nix;
   htop-example-settings = ./example-settings.nix;
+  settings-without-fields = ./settings-without-fields.nix;
 }