Fix tuples for bisect calls

[mjeronimo]

Python3 doesn't allow comparisons between 'NoneType' and some other types, like
float, generating the following error:

TypeError: '<' not supported between instances of 'float' and 'NoneType'

This error was encountered when calling bisect() with tuples that have a second element
as 'None' (one occurance is when displaying all thumbnails) where the corresponding element
type is int, float, etc. Instead, single element tuples are created, which result in the correct
comparison behavior.

Signed-off-by: Michael Jeronimo michael.jeronimo@openrobotics.org

[mabelzhang]

Thanks to your bag file with images (and your demo showing what that last button is. I know, or I should just read the wiki better), I can now reproduce the error!

One way to reproduce the error on master branch:
In Ubuntu 20.04 + ROS Noetic, which defaults to Python 3, run

$ rqt_bag

to bring up the GUI. Use the Load Bag button to load a bag file with image topics.
Right click in the timeline, check Thumbnails. Or use the Toggle Thumbnails button (last one on the right in the tool bar).
Errors, seems like one per image, will print in the terminal.
After the fix in this PR, images can now be displayed.

To make sure this still works for Python 2, I tested the branch in Ubuntu 18.04 + ROS Melodic as well, which defaults to Python 2. Things still work.

One thing I noticed though, which may or may not be related to this PR, is that Melodic shows thumbnails for these two topics

/camera/color/image_raw/compressed
/camera/depth/image_rect_raw/compressed

whereas Noetic does not. They are type sensor_msgs/CompressedImage.

Did you notice that? Is this something related that we can easily fix in this PR, or it's a separate issue?

Edit: I noticed another PR for image_helper, maybe it's fixed there.

[mjeronimo]

It should be a separate issue. This PR is targeted specifically at the "TypeError: '<' not supported between instances of 'float' and 'NoneType'" errors.

[mabelzhang]

Thanks! I'll create an issue for the other behavior - I don't see an existing ticket for it - for the sake of documenting it. We don't have to address it now.

[mabelzhang]

Issue created #74