Don't overwrite communities w/ blank extended communities list

See https://deepfield.atlassian.net/browse/SUP-282

https://deepfield.atlassian.net/browse/SUP-282?focusedCommentId=13589&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13589

"ext-community", w/ empty attr bufferm was overwriting "community" string.