Add check for Environment proto field before accessing in Dataflow provider

[modular-magician]

Adds an extra check to the Dataflow provider when accessing job proto fields; an edge case existed where the Environment proto field is nil, causing a nil-pointer exception when accessing sub-fields rather than an error that can be returned up the call stack. Checking for existence and bubbling an error up should eliminate that issue.

Validated the code change is propagated to the generated repos following https://googlecloudplatform.github.io/magic-modules/get-started/generate-providers/, also passed linting + unit tests + dataflow acceptance tests

Fixes #17046

Release Note Template for Downstream PRs (will be copied)

dataflow: fixes potential nil-pointer error if a job's Environment field is nil when reading job information in `google_dataflow_job`

Derived from GoogleCloudPlatform/magic-modules#10016

[github-actions]

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