AccountManager: Update display name only if it is not null.

During account linking initiated by the HttpLoginServlet, the code
overrides the user's display name with null.

This commit ensures that overriding the display name can only be done
when the provided value is not null.

Bug: Issue 3612
Change-Id: I8f47f5025349a244e2110aac902b44285cf8ad35
1 file changed