Fix crash with DEM and Camera

[scpeters]

🦟 Bug fix

Fixes crash introduced by #3257

Summary

While reviewing #3257, I didn't notice one instance in rendering/Heightmap.cc in which a SphericalCoordinates pointer will be passed in its default nullptr state when the surface type is EARTH_WGS84. It particularly causes a crash when loading a DEM in gzclient or loading a DEM with a camera sensor in gzserver. This adds a failing test with a CameraSensor and DEM in https://github.com/gazebosim/gazebo-classic/commit/9739cebbb4d56e427ff99edca2a97b7dc85f8670 and.a fix in https://github.com/gazebosim/gazebo-classic/commit/75f9e6cc73ccc4f999db8925939e3cf9c3237985.

Checklist

• [X] Signed all commits for DCO
• [X] 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

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