[BUG][Powershell] -WithHttpInfo ContentTypes issue

[EngineerCoding]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The -WithHttpInfo uses the $Response.Headers['Content-Type'] which may not always be set. Therefore this may result in Null which is not accepted by the DeserializeResponse function.

openapi-generator version

v5.0.0

Steps to reproduce

1. Generate any Powershell client
2. Run -WithHttpInfo on an endpoint that doesnt set the content type

Suggest a fix

(ApiClient.ps1)
Either let DeserializeResponse accept null values or use the following for the content type:

$contentTypes = $Response.Headers["Content-Type"]
if ($null -eq $contentTypes) {
    $contentTypes = [string[]]@()
}

# Use this in subsequent DeserializeResponse

[wing328]

Please submit a PR with the suggested fix and we'll review accordingly