Only generate a Go file if the file has some services

[kzys]

Otherwise, this generator creates a Go file with some import statements,
that causes "imported and not used" error.

Signed-off-by: Kazuyoshi Kato katokazu@amazon.com

[codecov-commenter]

Codecov Report

Merging #112 (f0fda53) into main (aa5c947) will decrease coverage by 1.97%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
- Coverage   63.87%   61.89%   -1.98%     
==========================================
  Files          11       12       +1     
  Lines        1049     1299     +250     
==========================================
+ Hits          670      804     +134     
- Misses        334      427      +93     
- Partials       45       68      +23     

Impacted Files	Coverage Δ
src/github.com/containerd/ttrpc/client.go	64.06% <0.00%> (-13.22%)	⬇️
src/github.com/containerd/ttrpc/channel.go	73.17% <0.00%> (-8.37%)	⬇️
src/github.com/containerd/ttrpc/server.go	73.79% <0.00%> (-2.84%)	⬇️
src/github.com/containerd/ttrpc/interceptor.go	100.00% <0.00%> (ø)
src/github.com/containerd/ttrpc/stream.go	81.48% <0.00%> (ø)
src/github.com/containerd/ttrpc/services.go	60.24% <0.00%> (+12.52%)	⬆️

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