Fix ChangePropertyKey whitespace quirks when moving subproperties

[timtebeek]

Summary

• Fixes Yaml/ChangePropertyKey - whitespace quirks #2608

When ChangePropertyKey moves a nested subproperty to a new top-level key (e.g. renaming foo.bar → bar), the DeletePropertyVisitor unconditionally cleared the surviving entry's prefix to "" when a mapping was reduced to one entry. This lost positional whitespace context, causing:

• A spurious leading newline when the property was in the first document entry
• A missing newline after --- in multi-document YAML (producing ---other: val instead of ---\nother: val)

The fix preserves the original first entry's prefix instead of clearing it, which carries the correct positional context in all cases.