modernize: use nullptr

src/ChatPage.h

src/LoginPage.cpp

src/LoginPage.h

src/MainWindow.h

src/RegisterPage.cpp

src/RegisterPage.h

src/RoomInfoListItem.h

src/RoomList.h

src/TextInputWidget.h

src/TopRoomBar.h

src/UserInfoWidget.h

src/UserSettingsPage.h

src/WelcomePage.h

src/dialogs/ImageOverlay.cpp

src/popups/ReplyPopup.cpp

src/popups/SuggestionsPopup.h

src/timeline/TimelineModel.h

src/timeline/TimelineViewManager.h

src/ui/Avatar.h

src/ui/Badge.h