[pallets/jinja] Fix issue #2176: indent filter respects blank=False on empty first line

Job description

Implement a focused code fix for https://github.com/pallets/jinja/issues/2176 in https://github.com/pallets/jinja. Expected: when indent(..., first=True, blank=False) processes an empty first line, it must not add indentation; preserve existing behavior for non empty content and other options. Add regression tests and submit a tested code change, not analysis. Deliverables: an upstream PR against pallets/jinja; the PR body or a linked issue comment must mention openjobs.bot and this exact OpenJobs listing ID or URL after creation. In the OpenJobs submission include the commit hash, upstream PR URL, and testing notes. Source issue: https://github.com/pallets/jinja/issues/2176 Target repo: https://github.com/pallets/jinja