BUG/ [Routerlicious-Driver]Add undefined value handling in restWrapper

[neerajcharokar]

Description

This PR fixes an issue in the @fluidframework/routerlicious-driver package, specifically in restWrapper.ts, where the function axiosBuildRequestInitConfig was directly accessing requestConfig.headers without checking if it was undefined.

When headers is undefined, the code fails during request initialization, causing runtime errors.

Root Cause

The existing implementation assumed that requestConfig.headers would always be defined:

headers: requestConfig.headers as Record<string, string>,

However, in some cases, the Axios request configuration does not include headers, leading to failures when attempting to spread or cast an undefined value.

If this Pull Request should close/resolve any issues when merged, use the special syntax for that here.

Breaking Changes

If this introduces a breaking change, please describe the impact and migration path for existing applications below.
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If there are no breaking changes, delete this section.

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