BUG: Add flags for PDF_QUALITY_LEVEL STREQUAL ""

When using the superbuild, the variable can default to "" -- in this case, use
the indended "Screen" default.

Change-Id: Ibb3bf2b9fefe75f7baeacc68fae54d4e03670c8e
Merge "BUG: Add flags for PDF_QUALITY_LEVEL STREQUAL """