fix(forms): make Validators.email support optional controls

[pmccloghrylaing]

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)

Don't appear to be any docs in need of updating.

PR Type

What kind of change does this PR introduce?

[x] 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?

Empty inputs with the email validator are invalid.

Issue Number: #16183

What is the new behavior?

Empty inputs with the email validator are valid unless they also have the required validator.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Applications will continue to work with this change, however any applications that relied on the email validator treating empty as invalid will need to be updated to also include the required validator.

Other information

#16276 and #16902 have been sitting unmerged for months - I've included changes from both in order to try to get this bug fix in.

[abdel-ships-it]

Someone please merge this PR.

[kara]

LGTM

[kara]

presubmit

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