bnc#862510: PPTX import: Wrong text color inside shape

When theme index is 0, it means it's unset so we should not
apply that.

(cherry picked from commit 89206c472ecf18bfde6824cea8004921cd404365)

Conflicts:
	sd/qa/unit/import-tests.cxx

Change-Id: I62a9cd2a9b4c19f5acffc334d5e8263fe24fc8fd