This PR was merged into the 4.4 branch.

Discussion
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[HttpClient] Handle requests with null body

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | /
| License       | MIT
| Doc PR        | /

since #45527 passing null to the `body` parameters leads to an exception (which [breaks async-aws](https://github.com/async-aws/aws/blob/09723ddca29b8d1d522426f81dd422373de1785f/src/Core/src/AbstractApi.php#L157))

> Argument #2 ($body) must be of type Closure, null given, called in /home/runner/work/aws/aws/vendor/symfony/http-client/CurlHttpClient.php on line 221

In curl client: `null` is not a string and `self::readRequestBody` expects a closure.

https://github.com/symfony/symfony/blob/08fa74a16c84895575e305b2a7ee3a03e371f79b/src/Symfony/Component/HttpClient/CurlHttpClient.php#L214-L221

In NativeClient, `getBodyAsString` will fail to return `null` because of the `string` return type.

Before #45527 null was converted to `""` thanks to the defaultOptions, but this is not the case anymore.

In many places, we check if the body is `!== ""` but rarely check if the body is null, this PR restores the original behaviors for the `body` parameters and converts nulls to `""`.

Commits
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39aec09 [HttpClient] Handle requests with null body