Fix crash in ResourceImporterTexture::_save_stex if webp disabled
Fix crash in ResourceImporterTexture::_save_stex if webp disabled

Fixes #51535.

(cherry picked from commit 79d4edfca7c9f7bb916058fabcdfafcc126fd5bb)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot