Fix work order, maintenance and schedule polymorphic serialization

[roncodes]

Problem

Editing a work order in the console and choosing a target asset or assignee failed on save with:

Assertion Failed: Model '@lid:work-order-…' has no attribute named 'target_type' defined.
Assertion Failed: Model '@lid:work-order-…' has no attribute named 'assignee_type' defined.

The work order, maintenance and maintenance schedule serializers read the polymorphic type with snapshot.attr('<key>_type'). Those models declare no such attribute, and Ember Data asserts on an undeclared one in debug builds.

Clearing a target or assignee also never reached the server. Ember Data only calls serializePolymorphicType for a related record that is present, so a removed relationship went out as null embedded data while <key>_uuid and <key>_type still pointed at the old record.

Change

• The three serializers read the type through snapshot.attributes(), which returns undefined for an undeclared attribute instead of asserting.
• A new clear-unset-polymorphic-relationships util sends each unset polymorphic relationship with its _uuid and _type set to null.
• Order, waypoint, device, equipment and entity are unchanged. Their models declare the type attribute, or never reach the hook.

Tests

• Regression tests serialize real work order, maintenance and schedule records, one relationship set and one unset.
• Unit tests cover the new util.
• Full dummy suite: 1056 tests, 0 failed.

Needed by fleetbase/fleetops#325.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cdbea0f) to head (130e11d).

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v0.2.1       #80   +/-   ##
================================================
  Coverage          100.00%   100.00%           
================================================
  Files                 115       116    +1     
  Lines                2922      2931    +9     
  Branches              697       698    +1     
================================================
+ Hits                 2922      2931    +9     

Flag	Coverage Δ
fleetops-data	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
addon/serializers/maintenance-schedule.js	100.00% <100.00%> (ø)
addon/serializers/maintenance.js	100.00% <100.00%> (ø)
addon/serializers/work-order.js	100.00% <100.00%> (ø)
...don/utils/clear-unset-polymorphic-relationships.js	100.00% <100.00%> (ø)

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