size: fix empty struct collection expressions

[jakezwang]

Collections of empty structs using the default map encoding currently produce invalid Msgsize expressions such as len(z.Named) * (1 + ), causing the generator to fail formatting otherwise valid Go input. Return the encoded header size alone when a struct has no field-size terms.

The regression generates and executes code for named and anonymous empty-struct slices and fixed, zero-length, and nested arrays. It checks the size bound and byte/stream round trips.

Fixes #101.

Validation:

• All native packages, including _generated, pass on Go 1.24.13 and pass with -race on Go 1.26.6.
• The new generator regression passes with -race on Go 1.24.13, 1.25.13, and 1.26.6.
• make prepare, formatting, and golangci-lint 2.9.0 pass.

Local validation ran on macOS arm64. TinyGo was unavailable, so tinygotest was not run. The available lint version was 2.9.0 rather than CI's pinned 2.5.0.

[klauspost]

lgtm