[Graph] Fix cend function

[Graph] Fix cend function

- Fix undefined behavior in cend() of \'graph_core.h\'

  The cend() function dereference node_list.cend(). This is undefined
  behavior because it accesses memory in an area that is not actually
  allowed. So changed cend() to operate normally through cbegin() + size.

  However, these changes also include problems. Usually, check that
  node_list is empty, and if it is, it will be treated as an exception.
  if use cbegin() and cend() when node_list is empty, there might be a
  problem. So, Added this caution at @note.

- Fixed a minor typo.

Related : #2086

**Self evaluation:**
1. Build test:   [X]Passed [ ]Failed [ ]Skipped
2. Run test:     [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: SeoHyungjun <hyungjun.seo@samsung.com>