Suggestions for default valued materials

[azeey]

Summary

The default value of a materials rgba is [1 1 1 1], so I thought we should use that when material.rgba is None. This also increases test coverage by testing default values of emission and specular. The documentation on material.rgba also states

Note that textures are applied in GL_MODULATE mode, meaning that the texture color and the color specified here are multiplied component-wise.
So, we should not discard the rgba values when setting textures.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #63 (8441891) into ahcorde/mjcf2sdf/material (5c9eb8a) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@                      Coverage Diff                      @@
##           ahcorde/mjcf2sdf/material      #63      +/-   ##
=============================================================
+ Coverage                      96.98%   97.20%   +0.22%     
=============================================================
  Files                             19       19              
  Lines                            828      824       -4     
=============================================================
- Hits                             803      801       -2     
+ Misses                            25       23       -2     

Impacted Files	Coverage Δ
...o_sdformat/mjcf_to_sdformat/converters/material.py	91.48% <100.00%> (+3.25%)	⬆️

---

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 5c9eb8a...8441891. Read the comment docs.