fix: handle null parent nodes in apply_templated_fields

[b-per]

Summary

• apply_templated_fields crashed with TypeError: argument of type 'NoneType' is not iterable when a job had job_completion_trigger_condition: null and --templated-fields included a dotted path into that field (e.g. job_completion_trigger_condition.condition.project_id)
• Fixed by treating a null parent node the same as a missing key — replacing it with an empty dict before navigating further
• Added a regression test covering this case

Context

Triggered when users template nested fields in the job completion trigger condition across multi-environment migrations, where some jobs have job_completion_trigger_condition: null and others do not.

Test plan

• uv run pytest tests/exporter/test_export.py — all 10 tests pass
• Run import-jobs --templated-fields with a template that includes job_completion_trigger_condition.condition.project_id against an account that has a mix of jobs with and without a job completion trigger condition

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
src/dbt_jobs_as_code
main.py	318	94	70%	131–134, 140, 180–181, 214–217, 223, 269, 292–295, 300–303, 314–322, 329–337, 341, 344–346, 419–424, 438–439, 457–458, 462–464, 502, 505, 513–526, 529–532, 573–579, 582, 586, 603, 605, 607, 609, 612–623, 627–628, 668–672, 686, 690, 692, 697–710, 717–718, 722
src/dbt_jobs_as_code/client
__init__.py	207	46	78%	17, 56–57, 65, 126–127, 150–151, 174–175, 190–191, 208–209, 225–226, 245, 255, 335–350, 373, 389–390, 400–412, 415–426, 431–440
src/dbt_jobs_as_code/cloud_yaml_mapping
change_set.py	235	30	87%	27–29, 59, 65–66, 155, 212, 224, 248, 252–253, 259–261, 276–279, 336–350, 403–425, 461–475
src/dbt_jobs_as_code/importer
__init__.py	27	7	74%	14–15, 22–27
src/dbt_jobs_as_code/schemas
__init__.py	23	2	91%	68, 78
common_types.py	60	3	95%	67–68, 91
config.py	14	1	93%	18
job.py	141	7	95%	252, 257, 273–283
TOTAL	1228	190	85%

Tests	Skipped	Failures	Errors	Time
212	0 💤	0 ❌	0 🔥	15.743s ⏱️