FBX Export: Make empty shape keys exportable

Currently all empty shape keys are not exported when exporting as FBX. The reason for this is that empty shape keys cause issues like crashing Unity. This can cause confusion because it is done without explanation.
This patch fixes this by making the shape keys technically empty while still keeping them intact and working as shape keys.

Reviewers: campbellbarton, mont29

Reviewed By: mont29

Tags: #bf_blender_2.8, #addons

Differential Revision: https://developer.blender.org/D4496