Leaving them at UUID.Zero meant that when a viewer 2 logged into a region that had been freshly created, it received UUID.Zero for these textures, and hence display the land as plain white.
On a simulator restart, the problem would go away since when the database adapators loaded the new region settings, RegionSettings itself has code to use default textures instead of UUID.Zero.
This commit resolves the problem by saving the default texture UUIDs instead of Zero.
However, we currently have to do this in a roundabout way by resaving once the RegionSettings have been created by the database for the first time. This needless complexity should be addressed.
This change will also have the effect of replacing any existing UUID.Zero terrain textures with the default ones.
However, this shouldn't have any effect since the UUID.Zeros were already being replaced in memory with those same UUIDs.
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