* Handle ConditionalStyle NumberFormat When Reading Xlsx File
ReadStyle in Reader/Xlsx/Styles.php expects numberFormat to be a string.
However, when reading conditional style in Xlsx file, NumberFormat
is actually a SimpleXMLElement, so is not handled correctly.
While testing this change, it turned out that reader always expects
that there is a "SharedString" portion of the XML, which is not
true for spreadsheets with no string data, which causes a
run-time message.
Likewise, when conditional number format is not one of the built-in
formats, a run-time message is issued because 'isset' is used
to determine existence rather than 'array_key_exists'.
The new workbook added to the testing data demonstrates both those
problems (prior to the code changes).
* Move Comment to Resolve Conflict
Github reports conflict involving placement of one comment statement.
* Respond to Scrutinizer Style Suggestion
Change detection for empty SimpleXMLElement.
Adrien Crivelli