handle nullptr SharedString in ScQueryEvaluator (tdf#149679)

This may be the case if the string comes from ScMatrix::Get()
for ScMatValType::Empty.

Change-Id: I35013449611bf7ffd1bc74e023d76597af010724
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/136347
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

sc/source/core/data/queryevaluator.cxx

@@ -491,8 +491,9 @@ std::pair<bool, bool> ScQueryEvaluator::compareByString(const ScQueryEntry& rEnt
                     const rtl_uString* pQuer = rItem.maString.getDataIgnoreCase();
                     const rtl_uString* pCellStr = rSource.getDataIgnoreCase();
 
-                    assert(pQuer != nullptr);
                     assert(pCellStr != nullptr);
+                    if (pQuer == nullptr)
+                        pQuer = svl::SharedString::getEmptyString().getDataIgnoreCase();
 
                     const sal_Int32 nIndex
                         = (rEntry.eOp == SC_ENDS_WITH || rEntry.eOp == SC_DOES_NOT_END_WITH)