The definitions of Get and Put imply, thru their schema, that
a representation could be empty:

Get Response is defined as:
  <wst:GetResponse ...>
    xs:any*
  </wst:GetResponse>

Put is defined as:
  <wst:Put Dialect="xs:anyURI"? ...>
    xs:any*
  </wst:Put>

Put Response is defined as:
  <wst:PutResponse ...>
    xs:any*
  </wst:PutResponse>

We then have text similar to this:
--
[Body]/wst:Put
    This REQUIRED element MUST have as its first child element, an element that
comprises the representation of the resource that is to be replaced. Additional
extension elements MAY be included after the element representing the resource. 
--

The MUST would seem to imply that some XML is required.  The schema seems
to imply that it could be empty.  The text about extensions makes it really
interesting since if the representation is empty then you can't tell if the
element that is present is the representation or an extension.  We have a
bit of ambiguity/confusion here.

Proposal:
Allow for empty representations, but leave it up to extensions to figure out
how to know when their elements are extensions vs part of the resource.

So, as an example, modify Get Response from:
[Body]/wst:GetResponse
    This REQUIRED element MUST have as its first child element, an element that
comprises the representation of the resource. Additional extension elements MAY
be included after the element representing the resource. 

to:
[Body]/wst:GetResponse
    This is a REQUIRED element. The resource representation, if any, MUST
be its first child element. Additional extension elements MAY be included
after the element representing the resource. If resource representation is
empty then the extension specification MUST include a mechanism to ensure the
extension elements are not mistakenly interpreted as the resource
representation.

Make similar mods to the other ops as needed.