Automatic generation of Timestamp for Positioned.fromMap if it is not…

[guplem]

Should fix #1392

The fromMap method of Positioned now checks if the timestamp exists, and if it does not exist, assume that the timestamp is "now".

Pre-launch Checklist

• I made sure the project builds.
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I rebased onto main.
• I added new tests to check the change I am making, or this PR does not need tests.
• I made sure all existing and new tests are passing.
• I ran dart format . and committed any changes.
• I ran flutter analyze and fixed any errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b311084) 94.73% compared to head (5162e24) 100.00%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #1411      +/-   ##
===========================================
+ Coverage   94.73%   100.00%   +5.26%     
===========================================
  Files           4        13       +9     
  Lines         114       246     +132     
===========================================
+ Hits          108       246     +138     
+ Misses          6         0       -6     

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[mvanbeusekom]

LGTM, thank you for this contribution