Make sure Dockerfile is not set to empty string

[SaschaSchwarze0]

Changes

When creating a build like this:

$ shp build create sample-go --source-url https://github.com/shipwright-io/sample-go --source-context-dir docker-build --strategy-name kaniko --output-image ... --output-credentials-secret ...

then, spec.dockerfile was set to "" which does not work. It is defined as a *string and therefore must be set to nil instead.

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Set a kind label on this PR
• Release notes block has been filled in, or marked NONE

Release Notes

When providing no value for the --dockerfile argument during build creation, then it is not anymore set to an empty string

[gabemontero]

/approve
/lgtm

thanks @SaschaSchwarze0

[openshift-ci]

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