software/theia: Fix null parameters templating

Before this commit, instance parameters that have a default null value
in JSON format were rendered as 'None' string in instance-theia.cfg.

In particular, Theia frontends were always requested with unsolvable
SLA parameters that prevented a frontend from ever being allocated:
```
sla-instance_guid = None
```
2 jobs for fix/simplehttpserver
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master