Every Resource schematron that is generated includes a global-1 constraint that looks like this:

 <sch:pattern>
 <sch:title>Global 1</sch:title>
 <sch:rule context="f:*">
 <sch:assert test="@value|f:*|h:div">global-1: All FHIR elements must have a @value or children</sch:assert>
 </sch:rule>
 </sch:pattern>

The context for this rule is f:* which means "any element (in the FHIR namespace)".

Applying that to the root element of a FHIR XML payload, it is looking for one of:
@value, f:*, or h:div which means: an attribute named "value", any sub-element (in the FHIR namespace), or a div tag for the narrative block XHTML subset.

This constraint does not allow for an empty Resource (e.g. Parameters).