[Fix] Don't destroy NULL windows if args.show == False

[bladrome]

Motivation

Run the demo code in remote machine without cam.

Modification

Add args.show checking in demo/foo.py

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1104 (2397c77) into master (bf691c0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1104   +/-   ##
=======================================
  Coverage   83.39%   83.39%           
=======================================
  Files         196      196           
  Lines       15142    15142           
  Branches     2736     2736           
=======================================
  Hits        12627    12627           
  Misses       1823     1823           
  Partials      692      692           

Flag	Coverage Δ
unittests	83.32% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bf691c0...2397c77. Read the comment docs.

[OpenMMLab-Assistant003]

Hi @bladrome！First of all, we want to express our gratitude for your significant PR in the MMPose project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

We would also like to invite you to join our Special Interest Group (SIG) private channel on Discord, where you can share your experiences, ideas, and build connections with like-minded peers. To join the SIG channel, simply message moderator— OpenMMLab on Discord or briefly share your open-source contributions in the #introductions channel and we will assist you. Look forward to seeing you there! Join us ：https://discord.gg/raweFPmdzG

If you are Chinese or have WeChat，welcome to join our community on WeChat. You can add our assistant ：openmmlabwx. Please add "mmsig + Github ID" as a remark when adding friends：）
Thank you again for your contribution❤ @bladrome