Applying terrain with one or more null material IDs is allowed, not caught by the validator

[cosmic-linden]

Test plan

See "Saving Composition" in https://github.com/secondlife/viewer/blob/develop/doc/testplans/pbr_terrain_composition.md

[cosmic-linden]

I'm going to just prevent setting the PBR materials from being set to null, and have them default to the blank material instead. See https://github.com/secondlife/viewer-private/issues/143 for selecting prettier default PBR terrain materials to show to the user.

[DmitrySProductengine]

Failed QA.
Verified on the Second Life Release 7.1.8.9103842320 (64bit) on Win10/OSX in the scope of IQA #231.

This ticket does not specify the exact test plan for verification.
I checked the test plan part (Featurettes Test Plan - 3. PBR Terrain) and everything is running correctly.
In step 7, where the checkbox is enabled, blank PBR materials are displayed.
However, the test plan does not describe the checks or behavior of blank or default PBR materials.

The reason for the failure was the different behavior of the viewer on Win and OSX:

1. OSX - the viewer does not allow the use of blank and default PBR materials. (perhaps this is the expected behavior of this ticket).
   7.1.8.9103842320 - OSX - default and blank PBR:

2. Win10 - the viewer does not allow the use of blank PBR materials. But it uses the default PBR materials (which look like blank ones).
   7.1.8.9103842320 - Win10 - blank PBR

7.1.8.9103842320 - Win10 - default PBR

Also, if possible, please add a test plan to check this ticket.

[cosmic-linden]

Sounds like the issue on Mac could be related to #989 ? cc: @brad-linden

Edit: Not related. Above ticket was a shader issue.

[cosmic-linden]

A test plan has been added as well.