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.

[SameerMesiah97]

Could we make this test exercise the filtering logic? Right now it only verifies the unconfigured case. Since the implementation now filters out empty entries, it woud be good to include input like "host1,,host2," (or even just ",") and assert that only the non-empty hosts are passed to AzureIdentityAuthenticationProvider.

[SameerMesiah97]

This could be clearer. Please see the below:

allowed_hosts = config.get("allowed_hosts", authority)
if not allowed_hosts:
    allowed_hosts = ""

allowed_hosts = [host for host in allowed_hosts.split(",") if host]