azurerm_api_management_api : Fix the error caused by api_type being specified as websocket but service_url is specified as an empty string

[sinbai]

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Description

The API behaves as follows:

• When api_type is specified as websocket but service_url is specified as an empty string(service_url = null or omit service_url attribute), the API returns error "WebSocket API type is not enabled for the service or sku".

• When api_type is specified as websocket and service_url is not specified, the API returns error "'serviceUrl' must not be empty."

Submit this PR to fix #29610 by not passing an empty string when service_url is not specified or specified as null. Also add the validation that service_url must be specified when api_type is specified as websocket.

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Testing

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TC test results:
https://hashicorp.teamcity.com/buildConfiguration/TF_AzureRM_AZURERM_SERVICE_PUBLIC_APIMANAGEMENT/380603?buildTab=tests

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_api_management_api - fix the error caused by api_type being specified as websocket but service_url is specified as an empty string [azurerm_api_management_api sends serviceUrl: "" when service_url is omitted/null, causing Azure 400 ValidationError for WebSocket APIs #29610]

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #29610

Note

If this PR changes meaningfully during the course of review please update the title and description as required.

[katbyte]

LGTM 🚜

[github-actions]

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