Avoid attempting to load cubemap when envMap is not set for probe light (fix #5610)

[mrxz]

Description:
In case no envMap was set for a probe light an attempt was still made to load it as a cube map. This resulted in a failed request and an error message in the console (see #5610)

This PR updates the updateProbeLight function to avoid this as well as clean up the caching logic. The cache would contain promises while the cube map was loading, but these never called the resolve method. This meant that if two probe lights would load the same envMap one of them would endlessly wait on a never-resolving Promise.

Fixes #5610

Changes proposed:

• Don't continue loading cube map when envMap is not set
• Make the cube map loading promise resolve the resulting LightProbe instance

[dmarcos]

Thank you very much!