Work around JRuby issue #1923.

String#start_with?() returns false on JRuby when used with an empty string. See
https://github.com/jruby/jruby/issues/1923 for more information.

lib/oga/xpath/evaluator.rb

@@ -855,7 +855,8 @@ module Oga
         haystack_str = on_call_string(context, haystack)
         needle_str   = on_call_string(context, needle)
 
-        return haystack_str.start_with?(needle_str)
+        # https://github.com/jruby/jruby/issues/1923
+        return needle_str.empty? || haystack_str.start_with?(needle_str)
       end
 
       ##