--hostname only works if --userdata already contains a hostname section

[tommack]

At the very least a warning would be helpful if --hostname is used but there is nothing to replace.

[rafaeleyng]

Could you elaborate? I had what seems to be the exact opposite problem. I've tried to set the hostname via --userdata to no avail. But --hostname worked just fine.

[killerwhile]

A bit more details: in image such as ubuntu 20.04, which doesn't include user-data file by default, if no user-data file is provided or the one provided doesn't include a hostname: xxx section, the parameter --hostname doesn't do anything.
flash is testing the presence of user-data file and replace an existing hostname: xxx directive to what is provided.