Fix java.util.NoSuchElementException: No value present

This fixes an issue where forUser.getUserName().get() could return
something that was unexpected leading to gerrit returning 500 errors.

Making it return null if it returns something unexpected fixes
the issue.

Bug: Issue 9820
Change-Id: If6c227d5134f81e7604b13800f90f785fa2ad206

src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java b/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
index b3c9062..fdac920 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
@@ -81,7 +81,7 @@
     }
     StringBuilder avatarUrl = new StringBuilder();
     String userReplacedAvatarURL = replaceInUrl(USER_PLACEHOLDER,
-        externalAvatarUrl, forUser.getUserName().get());
+        externalAvatarUrl, forUser.getUserName().orElse(null));
     avatarUrl.append(replaceInUrl(EMAIL_PLACEHOLDER, userReplacedAvatarURL,
         forUser.getAccount().getPreferredEmail()));
     if (imageSize > 0 && sizeParameter != null) {