fix(core): correctly concatinate static and dynamic binding to `class…

[mhevery]

…` when shadowed

Given:

<div class="s1" [class]="null" [ngClass]="exp">

Notice that [class] binding is not a string. As a result the existing logic would not concatenate [class] with class="s1". The resulting falsy value would than be sent to ngClass which would promptly clear all styles on the <div>

The new logic correctly handles falsy values for [class] bindings.

Fix #35335

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• 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
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[mhevery]

presubmit

[kara]

Thanks for picking this up! Some minor changes requested below.

• Can we also add some context to the commit message itself on what was broken and how we fixed it?
• In the commit, concatinate => concatenate

[kara]

LGTM, thanks for the great commit message

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