Remove obsolete @NotNull validation in case @NotEmpty is present

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Replace @NotNull with @NotEmpty where appropriate

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #588 from sonatype/fix-notempty-validation

REVIEW: @NotEmpty related fixes
Kill Nexus421PlainNotificationIT
Remove more check_smtp_settings leftovers in ui and testsuite
Require explicit bindings for core and plugins; this can help find broken/incorrect bindings.

http://google-guice.googlecode.com/git/latest-api-diffs/3.0/javadoc/com/google/inject/Binder.html#requireExplicitBindings()