[navigation] Add default render path when there is basePath

[SuZhou-Joe]

Description

As we will make dev tools as a full page modal when new nav toggle is turned on, we will use MemoryRouter to replace HashRouter in our page to avoid modifying the hosted page' hash. And this change will impact search relevance functionality.

And this PR adds a default path when there is no '/' route.

For more details please take a look on the comment: opensearch-project/OpenSearch-Dashboards#7938 (comment).

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[TackAdam]

LGTM. Tested with new navigation enabled and disabled with no problems.

[ps48]

Can we please test do previously supported flint URLs work as expected?

path={`/${basePath}/:dataSource`}
path={`/${basePath}/accelerate/:dataSource`}
path="/"
path={`/${basePath}`}

[TackAdam]

Implemented by : #387
Can be closed.

[joshuali925]

bug fixed by #387