TMDM-7749: "Error message when welcome page refreshes due to a statistics problem"

Don't throw exception when a data storage does not exist but return empty result (storage might not exist due to previous errors).
Minor javadoc formatting issue.
Missing isDebugEnabled() check in code.
(https://jira.talendforge.org/browse/TMDM-7749)
Merge branch 'maintenance/5.6'