Provide nonce as String, not Optional<String>, to Soy

Otherwise, when the nonce is available the index page fails to load,
with "500 Internal Server Error", due to the server-side exception
"IllegalArgumentException: Unable to convert param csp_nonce to a
SoyValue".

Noticed via a Google-internal integration test.  Tested by re-running
the same test with this change applied.

Google-Bug-Id: b/33429040
Change-Id: I98fc6ea267eff6a332e1345f79a4e7bbf51376a7

java/com/google/gitiles/Renderer.java

diff --git a/java/com/google/gitiles/Renderer.java b/java/com/google/gitiles/Renderer.java
index 3b85e3d..3bf9754 100644
--- a/java/com/google/gitiles/Renderer.java
+++ b/java/com/google/gitiles/Renderer.java
@@ -238,7 +238,7 @@
             .put("SITE_TITLE", siteTitle);
     Optional<String> nonce = req.map((r) -> (String) r.getAttribute("nonce"));
     if (nonce.isPresent() && nonce.get() != null) {
-      ij.put("csp_nonce", nonce);
+      ij.put("csp_nonce", nonce.get());
     }
     return getSauce().renderTemplate(templateName).setIj(ij.build());
   }