Do not check for labels in empty input while writing hostPrefix

[trivikr]

Issue #, if available:
Fixes CI failures in aws-sdk-js-v3 which started with https://github.com/aws/aws-sdk-js-v3/actions/runs/590870041

Description of changes:
Do not check for labels in empty input while writing hostPrefix.

Verified that gradle build is successful in aws-sdk-js-v3 after the change from this PR is published to local maven.

$ pwd
/home/trivikr/workspace/smithy-typescript

$ ./gradlew clean publishToMavenLoca
...
...
BUILD SUCCESSFUL in 3s
12 actionable tasks: 11 executed, 1 up-to-date

$ pwd
/home/trivikr/workspace/aws-sdk-js-v3/codegen

$ ./gradlew clean smithy-aws-typescript-codegen:build protocol-test-codegen:build -Plog-tests
...
...
BUILD SUCCESSFUL in 15s
13 actionable tasks: 13 executed

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