Fix empty string in attribute description in ldap

- if description is empty string, work with it like with null
- use in resource and group

(Cherry-pick of 6644819a70aec39db9402450b59050cae2260aa5)

Conflicts:
	perun-ldapc/src/main/java/cz/metacentrum/perun/ldapc/processor/impl/EventProcessorImpl.java
	perun-ldapc/src/main/java/cz/metacentrum/perun/ldapc/processor/impl/LdapConnectorImpl.java