Fix blank entry being interpreted as 0 offset.
Add test to ensure ongoing behaviour.
Removed some unnecessary graph update code.
Merge pull request #1167 from arjclark/fix_explicit_null_final_cp

cylc config: Fix explicit null final cycle being interpreted as 0 offset
Merge pull request #1168 from hjoliver/graph-cmd.decruft

Removed some unnecessary graph update code.