getSvxBrushItemFromSourceSet() is called once for each property of a
frame style that is set, and the "solid" fill-style is set by first
applying the BackColorRGB and then BackTransparency property.  So there
is an intermediate state that has fill-style NONE but a XFillColorItem
set - don't reset the color to white in that case!

Apparently writerfilter generally relies on the current reset-to-AUTO
implementation (loads of test failures), so for now limit the fix for
when we know that a frame style is imported from ODF.

(regression from 3d399b0e)

Change-Id: Ia987fe586ec819a1f3cd85acdbfb03c74ca0ec2c