Spotbugs issues (#4416)

* Spotbugs: Fixed comparing strings with equals instead of ==

* Spotbugs: Do not return null in @Nonnull marked method, instead return empty string.

* Added Suppress for Spotbugs

* Apply suggestions from code review

Co-Authored-By: Oleg Nenashev <o.v.nenashev@gmail.com>

* fixed javadoc

Co-authored-by: Oleg Nenashev <o.v.nenashev@gmail.com>