Common.soy: make explictely nullable the css parameter

To help with later changes in Soy, we are marking optional parameters
explicitely as nullable.

Change-Id: I94703c6fd28457a997a877fd0b88157e445f5095

resources/com/google/gitiles/templates/Common.soy

diff --git a/resources/com/google/gitiles/templates/Common.soy b/resources/com/google/gitiles/templates/Common.soy
index c5fb238..b47eb2c 100644
--- a/resources/com/google/gitiles/templates/Common.soy
+++ b/resources/com/google/gitiles/templates/Common.soy
@@ -24,7 +24,7 @@
       */
   {@param? customVariant: ?}  /** variant name for custom styling. */
   {@param breadcrumbs: ?}  /** navigation breadcrumbs for this page. */
-  {@param? css: list<?>}  /** optional list of CSS URLs to include. */
+  {@param? css: list<?>|null}  /** optional list of CSS URLs to include. */
   {@param? containerClass: ?}  /** optional class to append to the main container. */
   {@inject staticUrls: ?}
   {@inject SITE_TITLE: string}