Remove confusing code from DenseJacobianWriter

The code seemed to imply that its possible to call the Write
method with a null pointer which is never the case. There would
be no point to calling Write.

Thanks to Michael Vitus for pointing this out.

Change-Id: Ic9a276856d0a7e65d53a1cc8742d4831c1a52615

internal/ceres/dense_jacobian_writer.h

diff --git a/internal/ceres/dense_jacobian_writer.h b/internal/ceres/dense_jacobian_writer.h
index a0fec3f..1b04f38 100644
--- a/internal/ceres/dense_jacobian_writer.h
+++ b/internal/ceres/dense_jacobian_writer.h
@@ -70,10 +70,7 @@
              int residual_offset,
              double **jacobians,
              SparseMatrix* jacobian) {
-    DenseSparseMatrix* dense_jacobian;
-    if (jacobian != NULL) {
-      dense_jacobian = down_cast<DenseSparseMatrix*>(jacobian);
-    }
+    DenseSparseMatrix* dense_jacobian = down_cast<DenseSparseMatrix*>(jacobian);
     const ResidualBlock* residual_block =
         program_->residual_blocks()[residual_id];
     int num_parameter_blocks = residual_block->NumParameterBlocks();