fix: preserve view descriptions on scheme import

[xhon-pelushi]

Summary

• Preserve view descriptions when importing a table scheme via createFromScheme().
• Treat empty-string descriptions as intentional updates in ViewUpdateInput, with unit coverage.

Fixes #2478

Test plan

• Export a table with a view that has a description; import the scheme — view description should survive.
• Empty view description still clears/updates as intended.
• Run unit tests for ViewUpdateInput when PHP/Composer available.