When Node#getTextContent gets text content of node which is passed to XSLT
extension function, a string does not return but null returns.
Detailed Problem Statement Supporting Information:
When all the following conditions are satisfied, problem seems to occur.
1) A extension function is used in XSLT style sheet
2) A node which has text of child element(descendants) is passed as an
argument to the extension function.
3) In the extension function, getTextContent is called for the node