fix(file-history): guard update_components against nil render_data

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR adds a defensive nil guard in FileHistoryPanel:update_components() to prevent a crash when render_data is unexpectedly nil, and introduces a functional regression test suite to cover this and several related nil-guard fixes across the project.

Changes:

• Guard FileHistoryPanel:update_components() against self.render_data == nil before calling :destroy() / creating components.
• Add nil_guards_spec.lua functional tests covering multiple known nil-guard regressions, including the new FileHistoryPanel guard.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lua/diffview/scene/views/file_history/file_history_panel.lua	Adds an early return in update_components() when render_data is nil to prevent a crash.
lua/diffview/tests/functional/nil_guards_spec.lua	Adds functional regression tests for several nil-guard scenarios, including FileHistoryPanel:update_components() with nil render_data.

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