Default priorityClassName to blank string

[philnichol]

Issue #, if available:
Relates #249
Relates #252

Description of changes:
This fixes a bug I introduced in a previous PR (sorry!)
This must default to an empty string rather than null or you get:

Error: values don't meet the specifications of the schema(s) in the following chart(s):
s3-chart:
- deployment.priorityClassName: Invalid type. Expected: string, given: null

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-bot]

Hi @philnichol. Thanks for your PR.

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[RedbackThomson]

All good. Thanks

/lgtm

[ack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: philnichol, RedbackThomson

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