Fixed a bug where empty list of recipients of build notification messages...

Fixed a bug where empty list of recipients of build notification messages causes NPE in Mailer component.

Submitted by:  Vladimir Sizikov
Reviewed by:   Kohsuke

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git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@1124 71c3de6d-444a-0410-be80-ed276b4c234a