Issue description

This started in #17730, where I initially though that string null would return null, but then was corrected. However later it was found out that string function still can return null, so current signature is incorrect. There are 3 ways to fix it:

1. Just make return type string | null
2. Make return type string | null and fix cases like string null so they return null as expected
3. Leave return type to be string, but add more null checks so the result will be converted to empty string instead

Out of those three options while I like 2nd one, it seems to be impossible due to backwards compatibility, so I'd vote for the 3d option instead.

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

.NET SDK 9.0.0-rc.1.24431.7

Reproducible code snippet and actual behavior

type A() =
    override x.ToString() = null
let y = A() |> string // y is null

Possible workarounds

Leave as is, so if insightful user wants to do a null check, they should use `withNull:

type A() =
    override x.ToString() = null
let y = A() |> string |> withNull