TST: Barn-door testing of SaveButtons clicks

The canny_simple.py demo in viewer_examples/plugins has a save
button that raises an AttributeError in PIL because the button
click is passing 'False' to the SaveButtons.save_to_file function
when it expects a filename or None.

skimage/viewer/tests/test_widgets.py

@@ -101,6 +101,11 @@ def test_save_buttons():
     timer = QtCore.QTimer()
     timer.singleShot(100, QtGui.QApplication.quit)
 
+    # exercise the button clicks
+    sv.save_stack.click()
+    sv.save_file.click()
+
+    # call the save functions directly
     sv.save_to_stack()
     with expected_warnings(['precision loss']):
         sv.save_to_file(filename)

skimage/viewer/widgets/history.py

@@ -80,9 +80,9 @@ class SaveButtons(BaseWidget):
         notify(msg)
 
     def save_to_file(self, filename=None):
-        if filename is None:
+        if not filename:
             filename = dialogs.save_file_dialog()
-        if filename is None:
+        if not filename:
             return
         image = self.plugin.filtered_image
         if image.dtype == np.bool: