Fix LoadFromBag assumptions causing C++ exceptions during serialization (backport #438)

[mergify[bot]]

Description

• Not all bags have only GridMap messages
• Not all bags have GridMap on the right topic
• Add test for trying to load a grid map from a bag that doesn't contain it on the expected topic
• Add nullptr check on reader messages

Demo

Details

I uploaded a super small bagfile of two chatter topics. It's very small:

ryan@B650-970:~/Dev/ros2_ws/src/grid_map/grid_map_ros$ ll -h resource/double_chatter/
total 56K
drwxrwxr-x 2 ryan ryan 4.0K Feb 16 11:48 ./
drwxrwxr-x 3 ryan ryan 4.0K Feb 16 11:49 ../
-rw-r--r-- 1 ryan ryan  40K Feb 16 00:50 double_chatter.db3
-rw-rw-r-- 1 ryan ryan 4.8K Feb 16 11:48 metadata.yaml

It might be better to construct the bag file on the fly in the test case, but that would take much more time than what I did here.

Issue

Solves #401

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This is an automatic backport of pull request #438 done by Mergify.