JENKINS-20646 Summary is missing for the first build

- The build summary was displayed only if both current and previous reports were not null. Only current report should be tested at top-level, test of previous report is relevant only while processing the differences.
- Hardcoded "in" string on summary page added to localication Messages (English only).

- Create a new project.
- Process a first build.
- Show its summary.
- Verify the summary contains valid text.
- Process a second build.
- Show its summary.
- Verify the summary contains valid text.
- All tests passed.