rexml 3.4.3 Document.new no longer accepts strings with no root element

Followup to ManageIQ/manageiq-gems-pending#614

We generally create the xml document and add the root element and others afterwards.
This is ok, but we need to call REXML::Document.new(nil) instead.  In 3.4.3 and
future versions, it fails validation when the string provided to .new does not
contain a root element.

See the discussion in: https://www.github.com/ruby/rexml/pull/291

Author: jrafanie

app/controllers/miq_ae_class_controller.rb

@@ -401,7 +401,7 @@ def class_and_glyph(cls)
   end
 
   def build_details_grid(view, mode = true)
-    xml = REXML::Document.load("")
+    xml = REXML::Document.load(nil)
     xml << REXML::XMLDecl.new(1.0, "UTF-8")
 
     # Create root element