Fix an info message when using the convert CLI command with no args.input_format

[vinnamkim]

• Ticket no. 109749
• Current behavior: Source dataset format detected as 'None' <- 'None' should be replaced with the detected data format.

(datumaro) vinnamki@vinnamki:~/datumaro$ datum convert -i notebooks/d6-dice -f cvat -o ./ws_test/d6-dice-cvat --overwrite -- --save-media
2023-05-02 16:07:35,594 INFO: Source dataset format detected as 'None'
2023-05-02 16:07:49,679 INFO: Exporting the dataset
2023-05-02 16:07:51,706 INFO: Dataset exported to '/home/vinnamki/datumaro/ws_test/d6-dice-cvat' as 'cvat' 

• Expected behavior

(datumaro) vinnamki@vinnamki:~/datumaro$ datum convert -i notebooks/d6-dice -f cvat -o ./ws_test/d6-dice-cvat --overwrite -- --save-media
2023-05-02 16:07:35,594 INFO: Source dataset format detected as yolo
2023-05-02 16:07:49,679 INFO: Exporting the dataset
2023-05-02 16:07:51,706 INFO: Dataset exported to '/home/vinnamki/datumaro/ws_test/d6-dice-cvat' as 'cvat' 

• Add test_convert.py as well
• This will be merged into releases/1.2.0 to make datumaro==1.2.1.

How to test

I added test_convert.py to test the captured logs.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (2b7fb35) 78.70% compared to head (00eea45) 78.70%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/1.2.0     #982      +/-   ##
==================================================
- Coverage           78.70%   78.70%   -0.01%     
==================================================
  Files                 231      231              
  Lines               26420    26423       +3     
  Branches             5258     5259       +1     
==================================================
  Hits                20795    20795              
- Misses               4400     4403       +3     
  Partials             1225     1225              

Flag	Coverage Δ
macos-11_Python-3.8	77.72% <0.00%> (-0.02%)	⬇️
ubuntu-20.04_Python-3.8	78.67% <0.00%> (-0.02%)	⬇️
windows-2019_Python-3.8	78.58% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
datumaro/cli/commands/convert.py	20.96% <0.00%> (-1.07%)	⬇️

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