Ensure SyntaxError on empty XML in strict mode

Previously raised RuntimeError in MRI for zero-length docs.

Fixes #1349. Related to #1005 and 2c9b7f1

lib/nokogiri/xml/document.rb

@@ -45,7 +45,13 @@ def self.parse string_or_io, url = nil, encoding = nil, options = ParseOptions::
         # Give the options to the user
         yield options if block_given?
 
-        return new if !options.strict? && empty_doc?(string_or_io)
+        if empty_doc?(string_or_io)
+          if options.strict?
+            raise Nokogiri::XML::SyntaxError.new("Empty document")
+          else
+            return new
+          end
+        end
 
         doc = if string_or_io.respond_to?(:read)
           url ||= string_or_io.respond_to?(:path) ? string_or_io.path : nil

test/xml/test_document.rb

@@ -28,8 +28,13 @@ def test_dtd_with_empty_internal_subset
 
       # issue #1005
       def test_strict_parsing_empty_doc_should_raise_exception
-        assert_raises(SyntaxError) do
-          Nokogiri::XML(StringIO.new('')) { |c| c.strict }
+        ["", " "].each do |empty_string|
+          assert_raises(SyntaxError, "empty string '#{empty_string}' should raise a SyntaxError") do
+            Nokogiri::XML(empty_string) { |c| c.strict }
+          end
+          assert_raises(SyntaxError, "StringIO of '#{empty_string}' should raise a SyntaxError") do
+            Nokogiri::XML(StringIO.new(empty_string)) { |c| c.strict }
+          end
         end
       end