style: Avoid returning fmt::Errors for things that are not formatting errors

They are not. The serialization code already checks if the result is
empty, which can happen for other reasons. This makes the code a bit
more resilient to misuse.

Differential Revision: https://phabricator.services.mozilla.com/D115133

components/style/properties/properties.mako.rs

@@ -1486,13 +1486,13 @@ impl ShorthandId {
                 // shorthand, since it only accepts CSS-wide keywords (and
                 // variable references), which will be handled in
                 // get_shorthand_appendable_value.
-                Err(fmt::Error)
+                Ok(())
             }
             % for property in data.shorthands_except_all():
                 ShorthandId::${property.camel_case} => {
                     match shorthands::${property.ident}::LonghandsToSerialize::from_iter(declarations) {
                         Ok(longhands) => longhands.to_css(dest),
-                        Err(_) => Err(fmt::Error)
+                        Err(_) => Ok(())
                     }
                 },
             % endfor