Fix NPE when ReviewInput's message is empty

Bug: Issue 2877
Change-Id: I3d31af8cdcc26c2dc48c316542f55c402fd63832

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
index 0225387..43668e63 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
@@ -287,7 +287,7 @@
               "negative line number %d not allowed on %s",
               c.line, path));
         }
-        c.message = Strings.emptyToNull(c.message).trim();
+        c.message = Strings.nullToEmpty(c.message).trim();
         if (c.message.isEmpty()) {
           listItr.remove();
         }