[JavaScript] Avoid undefined headers with skipDefaultUserAgent enabled

[jakobw]

Prior to this commit, a JS client generated with skipDefaultUserAgent enabled results in a src/ApiClient.js file that doesn't define this.defaultHeaders. This breaks because this.defaultHeaders is later directly passed to a superagent request which expects an object and not undefined.

Fixes #20791.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

Ping for NodeJS/Javascript technical committee members as suggested in the checklist: @CodeNinjai, @frol, @cliffano.

[wing328]

looks good to me

will merge after tests pass

thanks for the fix

[jakobw]

@wing328 Great, thanks! The two CircleCI checks that are still listed under "pending" here seem to have passed 13h ago if I click into the job details. Not sure what's going on there.

[wing328]

agreed this pr is good to merge which i just did

thanks again for the quick fix