Remove unnecessary null pointer checks

[elfring]

👀 An extra null pointer check is not needed in functions like the following.

• cactus/cactus/graph/graph_core.cpp

  Lines 71 to 76 in 3227a7f

  	BufferDesc::~BufferDesc() {
  	if (pooled_data) {
  	delete[] pooled_data;
  	pooled_data = nullptr;
  	}
  	}

• cactus/cactus/graph/graph_core.cpp

  Lines 108 to 112 in 3227a7f

  	BufferDesc& BufferDesc::operator=(BufferDesc&& other) noexcept {
  	if (this != &other) {
  	if (pooled_data) {
  	delete[] pooled_data;
  	}