Mujoco plugin's cmake build fixed when no tests are define

[IceShuttle]

🦟 Bug fix

Fixes #

Summary

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)
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[azeey]

I have updated #811 a bit since. Can you check if you still have the issue? I'm not sure if this fix is necessary.

[IceShuttle]

I have updated #811 a bit since. Can you check if you still have the issue? I'm not sure if this fix is necessary.

The issue is still there as I cannot compile without this change