MarkdownToHtml#parse: Annotate lang parameter as @Nullable

The lang parameter may be null, but dereferencing it is
protected by a call to Strings.isNullOrEmpty.

Change-Id: Iab12d6c202fd179043932d1d382f83d0ef1d2397

java/com/google/gitiles/doc/MarkdownToHtml.java

diff --git a/java/com/google/gitiles/doc/MarkdownToHtml.java b/java/com/google/gitiles/doc/MarkdownToHtml.java
index 2f0f0ff..f6289e2 100644
--- a/java/com/google/gitiles/doc/MarkdownToHtml.java
+++ b/java/com/google/gitiles/doc/MarkdownToHtml.java
@@ -357,7 +357,7 @@
     }
   }
 
-  private List<ParseResult> parse(String lang, String text) {
+  private List<ParseResult> parse(@Nullable String lang, String text) {
     if (Strings.isNullOrEmpty(lang)) {
       return null;
     }