Fix another crash on whitespace #191

Author: onizet

src/Html2OpenXml/Expressions/TextExpression.cs

@@ -101,6 +101,7 @@ public override IEnumerable<OpenXmlElement> Interpret (ParsingContext context)
                 text = " " + text;
             }
             else if (startsWithSpace && prevIsPhrasing
+                && node.PreviousSibling!.TextContent.Length > 0
                 && !node.PreviousSibling!.TextContent[node.PreviousSibling.TextContent.Length - 1].IsWhiteSpaceCharacter())
             {
                 text = " " + text;

test/HtmlToOpenXml.Tests/WhitespaceTests.cs

@@ -40,6 +40,7 @@ public void ConsecutiveDivs_ReturnsMultipleParagraphs ()
         [TestCase("<span>   Hello\r\n\r\nWorld!   </span>")]
         [TestCase("<div><u>Hello World! </u></div>")]
         [TestCase("<hr>\n <span>Hello World!</span>")]
+        [TestCase("<p><span></span> Hello World!</p>")]
         public void Multiline_ReturnsCollapsedText (string html)
         {
             var elements = converter.Parse(html);