spacemanager: Fix precondition check for plpgsql language check

The previous expression would cause a failure as it didn't
return any rows if the language did not exist.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6479/
pom: enforce maven-release-plugin 2.4.2

the latest plugin broke git support.

Acked-by: Gerd Behrmann
Target: master, 2.7, 2.6
Require-book: no
Require-notes: no