Fix variables import with structured falsy values

[Leondon9]

Fix variable import handling for structured falsy values.

This keeps core CLI and airflowctl import semantics aligned when a variable entry is shaped like {"value": ..., "description": ...} and the value is falsy, such as an empty string, 0, false, or null.

It also makes airflowctl variables import reject non-object JSON input with the existing invalid-file error instead of failing later while iterating.

Tests:

• breeze run pytest airflow-ctl/tests/airflow_ctl/ctl/commands/test_variable_command.py -xvs
• breeze run pytest airflow-core/tests/unit/cli/commands/test_variable_command.py::TestCliVariables::test_variables_import_with_structured_falsy_values -xvs
• prek run ruff --files airflow-core/src/airflow/cli/commands/variable_command.py airflow-core/tests/unit/cli/commands/test_variable_command.py airflow-ctl/src/airflowctl/ctl/commands/variable_command.py airflow-ctl/tests/airflow_ctl/ctl/commands/test_variable_command.py

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Was generative AI tooling used to co-author this PR?

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

[Prab-27]

Do we need this ? I believe - It's related any major changes
WDYT ?

[bugraoz93]

We are generating this while releasing. Indeed it doesn't need for any PR. Seems we are missing comment on top of document

[Prab-27]

Happy to see it has already implemented ! @bugraoz93 , Do we also need to add a prek hook for it ?