PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (no public API or documented behavior changes)

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
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

A UrlTree containing an empty leading path segment serializes with two leading slashes. For
example, RouterLink commands such as ['/', '', 'attacker.example', 'collect'] produce
//attacker.example/collect. Browsers interpret that href as a protocol-relative external URL,
even though the commands describe an internal route. Preserved or merged query parameters are
also appended to that external URL.

Issue Number: #69700

What is the new behavior?

DefaultUrlSerializer removes empty leading path segments while serializing, so the example above
becomes /attacker.example/collect. This mirrors the parser's existing normalization of repeated
leading slashes and keeps the generated URL on the application's origin.

The regression test covers the command-array path and an appended query parameter. A differential
local harness also compared 4,096 ordinary command arrays with the released serializer; their
serialized output remained byte-for-byte identical.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Formatting and whitespace checks pass. The focused source harness covers ordinary paths, query
parameters, fragments, named outlets, multiple empty leading segments, and browser-origin
resolution.