Spring Avoid unnecessary Map wrapping in DefaultPathSegment

Avoid creating unnessary wrapper for empty map in DefaultPathSegment

Comment From: sbrannen

Thanks for the PR!

It turns out that your proposal introduces a bug, which in turn made me notice an existing bug.

Namely, any map returned by PathSegment#parameters() must be immutable, and this includes the shared empty map (EMPTY_PARAMS).

Comment From: sbrannen

Superseded by #27064