Don't allow NULL as email subject

See #54, release log:

• 07edb07 by @yegor256: #54 jcabi-parent 1.23
• 89a197d by @yegor256: #54 null checks
• 8d3fa62 by @yegor256: email updated
• 67ee623: updated merge/commanders list
• a555446: TEAM.md page updated

Released by Rultor 1.64.1, see build log