compute_asset_extname should explicitly return nil in else clause

[kenta-s]

As is

# Compute extname to append to asset path. Returns +nil+ if
# nothing should be added.
def compute_asset_extname(source, options = {})
  
  return if options[:extname] == false
  
  extname = options[:extname] || ASSET_EXTENSIONS[options[:type]]
  
  extname if extname && File.extname(source) != extname
end
end

Nothing specified as a return value when the last condition is false. An else clause should be here and explicitly return nil.