When doctype set to Engineering Report, an error is thrown: `'' is not a permitted subtype of Standard: reverting to 'implementation'`

[ronaldtse]

When the document is set to:

:doctype: engineering-report

This error shows. Clearly the document is not a "Standard".

Document Attributes: '' is not a permitted subtype of Standard: reverting to 'implementation'

[opoudjis]

The error is being falsely raised by the boilerplate processor; moreover, an empty subdoctype should not be triggering this warning to begin with.