fix: PhrasingElementExpression.cs - check empty string in text element

[galyni]

I had an error while parsing some (admittedly wrong) html, like the following (I reduced to a minimal example)

<span> <div> <br /> <div> <span>Texte</span> </div> </div> </span>

The error was :

Index was outside the bounds of the array

The submitted code fixed the issue without causing any apparent regression, but I must confess I don't have enough understanding of the internals to assure the modification is needed. I leave that to the appreciation of the reviewer. I figured that, as long as we access the string by index, we should check that it is not empty. My doubts are about what to do if the string is empty.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[onizet]

This is an amazing PR, simple but efficient!