Fix build error on missing action category

• in Studio, create a User Action
• Do not select category or attribute to Automation Chain
• Do not save
• Switch to another screen
• Discard changes
• Attempt Studio package update or try to download Studio package
  => Receive error http 500 when downloading package or performing Hot Reload from Dev Tools extension
  => Receive error package is null when updating Studio Package from Admin Center

It should build the package anyway with an empty action category