Fix msgraph/Power BI auth failure from empty allowed_hosts list

[davidnzhang]

What

KiotaRequestAdapterHook._build_request_adapter derives allowed_hosts from the connection's extra field and passes it to AzureIdentityAuthenticationProvider. When neither allowed_hosts nor authority is configured, allowed_hosts returned [""] rather than a true empty list.

When this value eventually flows through to kiota's AllowedHostsValidator.is_url_host_valid method, it fails to skip validation via the early return:

if not self.get_allowed_hosts():
    return True

This PR filters out blank strings so an unconfigured connection yields [] and correctly takes the early exit described above.

Why

A recent change in the microsoft-kiota-abstractions package (microsoft/kiota-python@8755f70) fixed the AllowedHostsValidator to actually enforce a list of allowed hosts. Previously, it only checked whether a URL was structurally valid and ignored the configured allowed_hosts set entirely. The is_url_host_valid method now correctly extracts the hostname from the URL and checks is against the allow list.

This seemed to break existing Power BI connections which did not configure allowed_hosts or authority (which is the default for Power BI connections), as the validator receives a list with an empty string instead of a true empty list, causing requests to fail validation.

Note that before the Microsoft change this was harmless, as the validator method never actually compared against the configured hosts (return all([scheme, netloc]) -> True). This PR restores kiota's "skip validation" pathway for the default case where no allowed_hosts are set.

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[SameerMesiah97]

Fix is fine but the logic could be clearer and the test is a bit too shallow.

CI needs to be triggerred as well.