Remove support for nil in workspaces_agent_config_version

Issue - do not support null on workspaces_agent_config_... (#493992 - closed)

What does this MR do and why?

Following the comment here, we will need to remove the support on nil workspaces_agent_config_version on workspaces table.

raise of RuntimeError, and an error message which says something like this:

'#workspaces_agent_config' cannot be called until '#workspaces_agent_config_version' is set by the 'before_validation' hook. To automatically set it, call 'workspace.valid?', which will call '#set_workspaces_agent_config_version'.

We also would like to uncomment the validation on workspaces_agent_config_version after 17.5

MR acceptance checklist

• Update the code to raise runtime error if nil
• uncomment validation on workspaces_agent_config_version
• add/update unit testing

Screenshots or screen recordings

We should be fine with the code change, for reason i described here with danger.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.