Fix wxCommandProcessor::IsDirty() for unsaved unmodified case.

IsDirty() still returned true even after undoing all the commands which was
wrong, as there was nothing to save in this case.

Closes #15722.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75309 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775