[Persistence] NPE when loading component without prior model state 210

The NPE exception is thrown due to no model existed. So MCT only loads models when they exist.
Specifically, BatteryModel is added for TimelineComponent, whose values are specified during creation. When loading older Timelines, the value for BatteryModel are the default values. Since these values are editable in infoView, users can then choose to edit or not. For the other circumstances, the same thing should apply.

Wrote or re-wrote unit tests (Y/N): N
Ran entire suite of unit tests successfully (Y/N): Y
Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): N
Code review date and attendees: Code review by pull request
Describe potential impact on other areas of the code: Persistence
Describe any test prerequisites for this fix (e.g., restart the database): N
For bugs marked "Regression - Yes", describe the cause of the regression:
Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N