Change REXML::Document.new("") to behave the same as REXML::Document.new(nil)

## Why?
XML without a root element is invalid XML, but to maintain compatibility,
we will change the behavior of `REXML::Document.new("")` to be the same as `REXML::Document.new(nil)`.

Author: naitoh

lib/rexml/document.rb

@@ -95,7 +95,7 @@ def initialize( source = nil, context = {} )
       @entity_expansion_text_limit = Security.entity_expansion_text_limit
       super()
       @context = context
-      return if source.nil?
+      return if source.nil? or source == ""
       if source.kind_of? Document
         @context = source.context
         super source

test/parse/test_element.rb

@@ -14,12 +14,12 @@ def parse(xml)
     class TestInvalid < self
       def test_top_level_no_tag
         exception = assert_raise(REXML::ParseException) do
-          parse("")
+          parse(" ")
         end
         assert_equal(<<-DETAIL.chomp, exception.to_s)
 Malformed XML: No root element
-Line: 0
-Position: 0
+Line: 1
+Position: 1
 Last 80 unconsumed characters:
 
         DETAIL

test/test_core.rb

@@ -1550,6 +1550,10 @@ def test_ticket_138
                    REXML::Document.new(doc.root.to_s).root.attributes.to_h)
     end
 
+    def test_empty_doc
+      assert(REXML::Document.new('').children.empty?)
+    end
+
     private
     def attribute(name, value)
       REXML::Attribute.new(name, value)