ButtomNaviationBar doesn't show single page pdf

[dongwooklee96]

Hi everyone
In case that a PDF file has only 1 page, BottomNavigationBar does not appear at version 2.0.1
I think it would be a side effect.
The code handling the display condition of BottomNavigationBar should be changed as below,
before

pdf_viewer/lib/src/viewer.dart

Line 265 in 43ae962

bottomNavigationBar: (widget.showNavigation && widget.document.count > 1)

after

bottomNavigationBar: (widget.showNavigation && widget.document.count >= 1) 

What do you think about it?

PR: #98