...ever since 035d20bd "INTEGRATION: CWS
aquafilepicker01".  Unclear whether this was written under the assumption that
m_aCurrentFilter is a pointer (which would explain the OSL_TRACE message talking
about "null"), or whether it really wanted to check for an empty string (which
the code acutally happened to do).  So lets keep the empty-string check in,
given it was in there ever since the code's introduction in 2007.

Change-Id: I9e48b6ceccaf069c6a6a88d3918ba88379a72497