Fix SemVer.ToString to handle v0 case

[weshaggard]

Since we are treating v0 versions as prerelease we need to make sure
we don't accidently start to add the bogus prerelease label in cases where
we call ToString() on the version.

[weshaggard]

This is a follow-up to #1972

[azure-sdk]

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[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[ghost]

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