Only close the cache db if it is not null.

This fixes a bug where the cache db is not closed if an exception occurs when opening it. This unhandled exception will cause Nextflow to fail and prevent the Global.cleanUp() from being called.

modules/nextflow/src/main/groovy/nextflow/Session.groovy

@@ -1226,7 +1226,7 @@ class Session implements ISession {
             log.warn("Failed to cleanup work dir: ${workDir.toUriString()}")
         }
         finally {
-            db.close()
+            db?.close()
         }
     }