Fix setting default value of request.PostData of HarEntry

[nsrCodes]

since har entry is of type [src]

interface Entry {
  request: Partial<HarRequest>;
}

so request.postData could have the value undefined

The current approach of setting the default value for this key will fail when postData is undefined

This then breaks the switch case inside the prepare method because there is no optional chaining

This PR is broken into two commits. If only either one feels sufficient, the changes can be cherry-picked or both can be squashed.

[CLAassistant]

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