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