minor Insight code cleanup

preparatory to other changes for UI testing work
Expose file annotation namespace in OMERO.insight
Expose file annotation namespace in OMERO.web
Remove ERROR log on non-CloseableServant (Fix #11378)

Since the refactoring the close methods out of
`AbstractAmdServant` and into `CloseableServant`,
an ERROR has been printed for each stateless
service.
Merge pull request #1395 from mtbc/insight-tidyup

minor Insight code cleanup
Modify empty/null namespace check as suggested by Jean-Marie
Merge pull request #1403 from joshmoore/rebased/dev_4_4/11378-closeable-servant

Remove ERROR log on non-CloseableServant (Fix #11378) (rebased onto dev_4_4)
Merge pull request #1397 from sbesson/fileannotation_ns

Expose file annotation namespace in the clients