CA-150952: properly check for XAPI session existence
CA-150952: properly check for XAPI session existence

Don't check if a XAPI session exists by comparing session with the empty
string because if a XAPI session does exist it returns a error string,
which evaluates to true.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

(cherry picked from commit 3fd6375cc13cb4106153b15c31c62580fd7dcc89)

kostaslamda/sm,xapi-project/sm,siddharthv/sm,geosharath/sm,pritha-srivastava/sm,martinjorge/sm,geosharath/sm,geosharath/sm,robertbreker/sm,kostaslamda/sm,chandrikas/sm,xapi-project/sm,pritha-srivastava/sm,siddharthv/sm,germanop/sm,pritha-srivastava/sm,djs55/sm,kostaslamda/sm,djs55/sm,BobBall/sm,malcolmcrossley/sm,BobBall/sm,BobBall/sm,germanop/sm,robertbreker/sm,siddharthv/sm,robertbreker/sm,martinjorge/sm,malcolmcrossley/sm,malcolmcrossley/sm,djs55/sm,martinjorge/sm,chandrikas/sm,germanop/sm,xapi-project/sm,chandrikas/sm