Clarify nullable guidance for Signal Forms by distinguishing between required and optional fields.

Optional numeric fields may validly use null to represent an unset or empty value.

This updates outdated guidance in the Signal Forms reference that previously discouraged using null for all field types.

Changes include:

• clarifying that null should be avoided for required fields,
• documenting that optional fields may use null,
• updating numeric field examples,
• improving nullable model examples,
• updating the common pitfalls table,
• softening overly strict nullable input wording.

Fixes #68197

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 (not applicable for docs-only changes)
• Docs have been added / updated

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?

The Signal Forms reference currently states that null should not be used as a value or type for any fields, including numeric fields.

This guidance is outdated for newer Angular versions where optional fields (including numeric fields) may validly use null.

Issue Number: #68197

What is the new behavior?

The documentation now:

• distinguishes between required and optional fields,
• explains when null is appropriate,
• updates examples and pitfalls guidance accordingly.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This is a documentation-only update.