fix(@angular-devkit/build-angular): ensure that externalMetadata is defined

[cexbrayat]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

In v17.0.0-rc.1, ng serve might fail with: Cannot read properties of undefined (reading 'explicit') because result.externalMetadata is optional (see https://github.com/angular/angular-cli/blob/main/packages/angular_devkit/build_angular/src/tools/esbuild/bundler-execution-result.ts#L33)

What is the new behavior?

This commit guards the access and should fix the issue related to the fix 9768c18

Does this PR introduce a breaking change?

• Yes
• No

Other information

[alan-agius4]

LGTM

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}