Fix build event payload serialization when job dates are null

[slayton]

Fixes #21.

Build event payload creation can fail when JobModel is serialized without createdDate or updatedDate values. Those fields are only populated for item create/update events, but Jackson still calls their getters while serializing build events. The existing defensive-copy getters assumed the dates were non-null, causing a NullPointerException and resulting in an empty HTTP request body.

This change makes the createdDate and updatedDate accessors null-safe while preserving defensive copies when values are present.

Testing done

• Added unit coverage for initial null createdDate and updatedDate values.
• Added regression coverage that a build event serializes with a non-empty JSON body and omits null job date fields.
• Ran git diff --check HEAD~1 HEAD.

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