Avoid throw an first chance XmlException in ISymbolExtensions.GetDocumentationComment() when xmlText is empty?

[yyjdelete]

Version Used:
master-vs-deps(9401294)
Related to #36028

Steps to Reproduce:

1. Hover cursor on any params of non-override methods with document, or on any local vars in VS.

Expected Behavior:
No first chance exceptions should be throw.

Actual Behavior:
An XmlException is throw and catched. It's useless when xmlText is empty.

[KirillOsenkov]

This is a recent regression and is quite annoying as it happens on every hover.

[KirillOsenkov]

Note that there is potentially a second issue, not sure if unrelated: the XmlException can also have the message: "There are multiple root elements."

This can happen when you call XElement.Parse on text such as:

      <summary> Gets the solid fill color that has a hexadecimal value of #FFFFEBCD. </summary>
      <returns>A solid fill color.</returns>
    

There needs to be a better way to parse multiple XML elements like above, potentially use what the compiler does, or wrap the text with a fake root element.

You decide whether a separate issue should be filed for the above or it can be fixed as part of the fix for this issue.