[Platform] Avoid NPE on getMySandbox

Remove risk of hitting NPE on getMySandbox in
PlatformImpl. Stored reference to sandbox is
obtained during call to getBootstrapComponents,
but if that has not been called when getMySandbox
is needed, an NPE occurs.

This is significant for nasa/MCT-Plugins#134,
which requires some deployment-specific additions
to My Sandbox; since the most obvious place to
do this is at bootstrapping time, e.g. before
or during the first call to PlatformImpl's
getBootstrapComponents.

```
 Wrote or re-wrote unit tests (Y/N): N
 Ran entire suite of unit tests successfully (Y/N): N
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): N/A
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Self-review by pull request
 Describe potential impact on other areas of the code: N/A
 Describe any test prerequisites for this fix (e.g., restart the database): N/A
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N
```

No unit test added (one-line fix to support non-platform issue)