require "rexml"
REXML::Document.new("") # no error
REXML::Document.new("<?xml version='1.0'?>") # no error

https://www.w3.org/TR/2006/REC-xml11-20060816/#sec-well-formed

[Definition: A textual object is a well-formed XML document if:]

• Taken as a whole, it matches the production labeled document.
• It meets all the well-formedness constraints given in this specification.
• Each of the parsed entities
  which is referenced directly or indirectly within the document is well-formed.

Document

[1] document ::= ( prolog element Misc* ) - ( Char* RestrictedChar Char* )

Should we report an error for no root input?