MarkdownToHtml#parse: Return empty list instead of null

Change-Id: I579d9893fdebdce489254f7504a840748dd237da

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 e45503d..ec9f058 100644
--- a/java/com/google/gitiles/doc/MarkdownToHtml.java
+++ b/java/com/google/gitiles/doc/MarkdownToHtml.java
@@ -24,6 +24,7 @@
 import com.google.gitiles.ThreadSafePrettifyParser;
 import com.google.gitiles.doc.html.HtmlBuilder;
 import com.google.gitiles.doc.html.SoyHtmlBuilder;
+import java.util.Collections;
 import java.util.List;
 import javax.annotation.Nullable;
 import org.commonmark.ext.gfm.strikethrough.Strikethrough;
@@ -319,7 +320,7 @@
     html.open("pre").attribute("class", "code");
     text = printLeadingBlankLines(text);
     List<ParseResult> parsed = parse(lang, text);
-    if (parsed != null) {
+    if (!parsed.isEmpty()) {
       int last = 0;
       for (ParseResult r : parsed) {
         span(null, text, last, r.getOffset());
@@ -357,12 +358,12 @@
 
   private List<ParseResult> parse(@Nullable String lang, String text) {
     if (Strings.isNullOrEmpty(lang)) {
-      return null;
+      return Collections.emptyList();
     }
     try {
       return ThreadSafePrettifyParser.INSTANCE.parse(lang, text);
     } catch (StackOverflowError e) {
-      return null;
+      return Collections.emptyList();
     }
   }