Allow for allow_null_dvce_tstamps var

[agnessnowplow]

Description

Some of our trackers (e.g specific older versions of the AMP tracker or the Pixel tracker) do not send any value for dvce_created_tstamp and dvce_sent_tstamp. While the snowplow sessionization logic adds a filter to prevent potentially large table scans and reprocessing old sessions that are sent exceptionally late (e.g due to a bot) for such cases this should not matter as there is no retry on those trackers: events are sent right after tracking them or not at all.

This PR allows for setting a variable to coalesce the value with collector_tstamp, which should be available in all trackers.

This should not be a breaking change as it is added as an optional parameter. So far it has been tested locally and in this Unified PR.

What type of PR is this? (check all applicable)

• [ ] 🍕 Feature
• [x] 🐛 Bug Fix
• [ ] 📝 Documentation Update
• [ ] 🎨 Style
• [ ] 🧑‍💻 Code Refactor
• [ ] 🔥 Performance Improvements
• [ ] ✅ Test
• [ ] 🤖 Build
• [ ] 🔁 CI
• [ ] 📦 Chore (Release)
• [ ] ⏩ Revert

Related Tickets & Documents

Checklist

• [ ] 💣 Is your change a breaking change?
• [ ] 📖 I have updated the CHANGELOG.md

Added tests?

• [ ] 👍 yes
• [ ] 🙅 no, because they aren't needed
• [ ] 🙋 no, because I need help

Added to documentation?

• [ ] 📓 internal package docs (ymls, macros, readme, if applicable)
• [ ] 📕 I have raised a Snowplow documentation PR if applicable (Link here if required)
• [ ] 🙅 no documentation needed

[optional] Are there any post-deployment tasks we need to perform?