[Fix] skip joints that should be hidden in visualization

[womeier]

fix for issue #1227

joints with color=None should be skipped according to the documentation:

mmpose/mmpose/core/visualization/image.py

Line 126 in c09a693

pose_kpt_color (np.array[Nx3]`): Color of N keypoints. If None,

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.

[CLAassistant]

All committers have signed the CLA.

[womeier]

just noticed, that the same would happen with the 3d version, you may want to look at that too.

[codecov]

Codecov Report

Merging #1228 (213a499) into dev-0.25 (c09a693) will decrease coverage by 0.00%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##           dev-0.25    #1228      +/-   ##
============================================
- Coverage     83.44%   83.43%   -0.01%     
============================================
  Files           205      205              
  Lines         16625    16629       +4     
  Branches       2976     2976              
============================================
+ Hits          13872    13875       +3     
- Misses         2001     2002       +1     
  Partials        752      752              

Flag	Coverage Δ
unittests	83.36% <84.61%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/core/visualization/image.py	67.14% <84.61%> (+0.32%)	⬆️
mmpose/datasets/builder.py	78.94% <0.00%> (-0.79%)	⬇️

---

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 c09a693...213a499. Read the comment docs.

[womeier]

fixed the test

[OpenMMLab-Assistant003]

Hi @womeier！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❤ @womeier