PageText toString sometimes empty although getPageLines is not empty

[gtache]

Apparently, pageLines can be empty while getPageLines manages to retrieve the lines (therefore sortedLines and/or optionalPageLines are filled).
A simple fix would be to use getPageLines in toString. It would also arguably be more correct given that we could expect the string to be sorted.
The other methods using pageLines (applyTextTransform, clearSelected, etc) still work with an empty pageLines given that they also use sortedLines and optionalPageLines.

[pcorless]

This fix seems reasonable.