[mlir][sparse] fix typo that cause windows build failure.

[Peiming]

The patch was reverted before the fix, will submit another patch.

mlir/lib/Dialect/SparseTensor/Transforms/CodegenUtils.cpp

Operation *SparseTensorLoopEmitter::enterLoopOverTensorAtDim(
     OpBuilder &builder, Location loc, size_t tid, size_t dim,
     MutableArrayRef<Value> reduc, bool isParallel, ArrayRef<size_t> extraTids,
     ArrayRef<size_t> extraDims) {
 
   assert(dimTypes[tid].size() > dim);
   // We can not re-enter the same level.
   assert(!coord[tid][dim]);
   // TODO: support multiple return on parallel for?
-  assert(!isParallel || reduc.empty() <= 1);
+  assert(!isParallel || reduc.size() <= 1);
 
   Value step = constantIndex(builder, loc, 1);
   auto dimType = dimTypes[tid][dim];
   bool isSparseInput = isCompressedDLT(dimType) || isSingletonDLT(dimType);
   assert(isDenseDLT(dimType) || isCompressedDLT(dimType) ||
          isSingletonDLT(dimType));
 
   Value lo = isSparseInput ? pidxs[tid][dim]      // current offset