azurerm_stream_analytics_output_mssql: Username and password parameters optional if MSI authentication

[marcgs]

username and password parameters are optional when authenticationMode is MSI (Managed Service Identity).

Acceptance tests were adapted and run successfully:

TF_ACC=1 go test -v ./internal/services/streamanalytics -run=TestAccStreamAnalyticsOutputSql -timeout 20m -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccStreamAnalyticsOutputSql_basic
=== PAUSE TestAccStreamAnalyticsOutputSql_basic
=== RUN   TestAccStreamAnalyticsOutputSql_update
=== PAUSE TestAccStreamAnalyticsOutputSql_update
=== RUN   TestAccStreamAnalyticsOutputSql_authenticationModeMsi
=== PAUSE TestAccStreamAnalyticsOutputSql_authenticationModeMsi
=== RUN   TestAccStreamAnalyticsOutputSql_requiresImport
=== PAUSE TestAccStreamAnalyticsOutputSql_requiresImport
=== RUN   TestAccStreamAnalyticsOutputSql_maxBatchCountAndMaxWriterCount
=== PAUSE TestAccStreamAnalyticsOutputSql_maxBatchCountAndMaxWriterCount
=== CONT  TestAccStreamAnalyticsOutputSql_basic
=== CONT  TestAccStreamAnalyticsOutputSql_requiresImport
=== CONT  TestAccStreamAnalyticsOutputSql_maxBatchCountAndMaxWriterCount
=== CONT  TestAccStreamAnalyticsOutputSql_authenticationModeMsi
=== CONT  TestAccStreamAnalyticsOutputSql_update
--- PASS: TestAccStreamAnalyticsOutputSql_basic (281.75s)
--- PASS: TestAccStreamAnalyticsOutputSql_authenticationModeMsi (322.58s)
--- PASS: TestAccStreamAnalyticsOutputSql_requiresImport (348.95s)
--- PASS: TestAccStreamAnalyticsOutputSql_update (367.62s)
--- PASS: TestAccStreamAnalyticsOutputSql_maxBatchCountAndMaxWriterCount (534.90s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/streamanalytics       537.343s

[katbyte]

LGTM 🌩️

[github-actions]

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