Fix explore command without project

[sooahleex]

Summary

• Fixed the problem datum explore not available on datasets #1252

How to test

• Add unit test for without project

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]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (0814dda) 80.65% compared to head (10799d1) 80.65%.

Files	Patch %	Lines
src/datumaro/cli/commands/explore.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1271   +/-   ##
========================================
  Coverage    80.65%   80.65%           
========================================
  Files          270      270           
  Lines        30371    30369    -2     
  Branches      6166     6167    +1     
========================================
- Hits         24496    24495    -1     
+ Misses        4492     4491    -1     
  Partials      1383     1383           

Flag	Coverage Δ
ubuntu-20.04_Python-3.10	80.64% <0.00%> (+<0.01%)	⬆️
windows-2022_Python-3.10	80.63% <0.00%> (+<0.01%)	⬆️

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