Warn and skip adding normal cloud if it is empty

[taketwo]

Currently the PCLVisualizer::addPointCloudNormals() function does not check if the supplied normal cloud is empty. While perhaps logically it's okay to add an empty cloud, the implementation of the function can not deal with empty clouds (see this line, size_t underflows and nr_normals becomes a ridiculously large number, bad alloc follows). This patch adds an a warning and an early return to avoid issues.