Make report formatters handle nil `Changes`

[devblac]

Make report formatters handle nil Changes

Context

I hit a panic when calling the report formatters with a Result that didn’t populate Changes (early exit case). The CLI swallowed it, but a direct library call crashes.
 

Problem

FormatJSON, FormatHTML, and FormatText assume result.Changes is non-nil and dereference it unconditionally.
 

Expected behavior

Passing a Result with a nil Changes should return a valid string without panicking, treating missing changes as empty.
 

Scope / non-goals

• Don’t alter the output shape when Changes is present; only guard the nil case.
• No formatter redesigns.
   

Acceptance criteria

• All three formatters return without panic when Changes is nil.
• Existing formatter snapshots stay identical when Changes is set.
• Tests cover the nil-Changes scenario for JSON, HTML, and text outputs.
   

Hints

• internal/report/json.go, html.go, text.go and their tests.