Fix state description of CastTile

The cast tile was concatenating empty string (with commas) for its state
description, leading to bad state description.

Instead, make sure to only add non empty strings and then concatenate
what we actually have.

Test: manual, talkback
Test: atest CastTile
Flag: EXEMPT bugfix
Fixes: 412321774
Change-Id: I26fbdab916e2e05fdbc76d49affff3978b2d2231