Improve semantic errors for allShortestPaths
Add semantic checking of merge actions
Add check that var length rels are not used in updates
Fix comparisons against null
Merge pull request #1277 from cleishm/semantic_checks

Semantic checks
Merge pull request #1265 from cleishm/shortestpath_checks

Improve semantic errors for allShortestPaths
Merge pull request #1278 from systay/null_comparisons

Fix comparisons against null