[JENKINS-21258] Graphical artefacts in the trend graphs

- If a language newly appears/disappears in/from SLOCCount report the trend graphs will contain graphical artifacts. The issue is visible mainly in the delta trend but it is (and always was) present in the absolute one too. The code doesn't consider that currently processed build may contain different languages than the previous and next ones.
- The issue fixed by adding zero or negative lines count for all known languages that are not present in the particular build but may be present in the previous or next ones.