Fix forc-fmt empty if-body + else collapsing onto one line

[Dnreikronos]

Description

Fixes #7625

forc-fmt collapses empty if-body + else onto a single line:

// Before (wrong):
if __size_of::<T>() == 0 {    } else if __is_reference_type::<T>() {

// After (correct):
if __size_of::<T>() == 0 {
} else if __is_reference_type::<T>() {

Root cause: In format_then_block, when the then-block is empty and has an else clause, block_unindent() was called but no newline was written — so the closing } from format_else_opt ended up on the same line.

Fix: Write a newline after the empty body when an else clause exists, and skip block_unindent since close_curly_brace in format_else_opt already handles unindentation. Guarded with LineStyle::Inline to avoid injecting newlines during width measurement probes.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to formatting behavior for a specific if shape (empty then-block with else/else if) and add regression tests; no runtime/compiler semantics are affected.

Overview
Fixes forc-fmt output so an empty if body followed by else/else if no longer collapses onto a single line by emitting a newline (except during inline width-probe formatting).

Adds regression coverage for this case (including a long-condition else variant) in both expression-level and integration formatter tests; includes a small reformat-only adjustment in sway-lib-std (raw_ptr.write) to match the corrected style.

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