Removed nullability checks from FormUrlEncodedContent ctor

[ManickaP]

As @stephentoub suggested, we can just avoid the problem by temporarily disabling nullability checks for the offending ctor.
Works with all following options:

new FormUrlEncodedContent(new Dictionary<string, string>());
new FormUrlEncodedContent(new Dictionary<string, string?>());
new FormUrlEncodedContent(new List<KeyValuePair<string?, string?>>());
new FormUrlEncodedContent(new List<KeyValuePair<string, string?>>());
new FormUrlEncodedContent(new List<KeyValuePair<string, string>>());

But still disallows passing in null itself:

new FormUrlEncodedContent(null); // warning CS8625: Cannot convert null literal to non-nullable reference type.

Fixes #38494
Closes #44458

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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Issue Details

---

As @stephentoub suggested, we can just avoid the problem by temporarily disabling nullability checks for the offending ctor.
Works with all following options:

new FormUrlEncodedContent(new Dictionary<string, string>());
new FormUrlEncodedContent(new Dictionary<string, string?>());
new FormUrlEncodedContent(new List<KeyValuePair<string?, string?>>());
new FormUrlEncodedContent(new List<KeyValuePair<string, string?>>());
new FormUrlEncodedContent(new List<KeyValuePair<string, string>>());

But still disallows passing in null itself:

new FormUrlEncodedContent(null); // warning CS8625: Cannot convert null literal to non-nullable reference type.

Fixes #38494
Closes #44458

Author:	ManickaP
Assignees:	-
Labels:	area-System.Net.Http, new-api-needs-documentation
Milestone:	-

[MihaZupan]

Very cool trick