Don't crash on bin/importmap pin with no arguments

[ajaynomics]

What

bin/importmap pin with no package arguments raised NoMethodError: undefined method 'each' for nil instead of printing a friendly message.

Why

On a pin with no packages, the JSPM service returns a 200 whose body has no map.imports. Importmap::Packager#extract_parsed_response builds { imports: parsed.dig("map", "imports") }, so the result is a truthy hash whose :imports value is nil. Importmap::Commands#for_each_import only guarded against a nil response, so the truthy hash sailed past the if response check straight into nil.each and crashed.

How

Tighten the guard from if response to if response && response[:imports].present?. An empty/missing-imports result now routes through the existing handle_package_not_found, giving the same UX as pinning an unknown package. This is consistent with the .blank? usage already present in that method, and stays a single line.

I considered the alternative of returning nil from extract_parsed_response when imports is nil (pushing the guard down to where the { imports: nil } shape is constructed). I kept the guard at the call site since for_each_import is the only consumer and the one-line check reads clearly there, but I'm happy to move it if preferred.

Test

Added regression tests in the existing house style (real bin/importmap subprocess against live jspm.io, matching the md5@2.2.0/pristine tests) asserting that both bin/importmap pin and bin/importmap unpin with no arguments print Couldn't find any packages instead of raising. Both fail on main with the NoMethodError and pass with this patch — the same for_each_import guard protects both commands.

Fixes #317