Remove graph % if there is no data to compare in previous cycle

## Bug Description

Under https://github.com/google/site-kit-wp/issues/10907 , we fixed the pie chart to not show any % for when there is no data to compare in the previous cycle.

It also makes sense to remove the % above the graph.
Refer to the attached screenshot.

## Steps to reproduce

Use the QAB of https://github.com/google/site-kit-wp/issues/10907

## Screenshots

## Additional Context

- PHP Version:
- OS: [e.g. iOS]
- Browser: [e.g. chrome, safari]
- Plugin Version: [e.g. 22]
- Device: [e.g. iPhone6]

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## Acceptance criteria

*

## Implementation Brief

* [ ]

### Test Coverage

*

## QA Brief

*

## Changelog entry

*

Start with the QAB for issue 10907 and compare its pie-chart fix with the graph shown in the attached screenshot. Trace the graph's percentage display and determine how it behaves when there is no data to compare in the previous cycle. Done means the percentage above the graph is absent in that case, while normal comparisons remain unchanged.

Written by the indexing model from the issue text.