Fix issue 10505 --- string function returns null on null paramete…

…r of type `string`.

src/fsharp/FSharp.Core/prim-types.fs

@@ -4509,7 +4509,10 @@ namespace Microsoft.FSharp.Core
         [<CompiledName("ToString")>]
         let inline string (value: 'T) = 
              anyToString "" value
-             when 'T : string     = (# "" value : string #)     // force no-op
+
+             when 'T : string =
+                if value = unsafeDefault<'T> then ""
+                else (# "" value : string #)     // force no-op
 
              // Using 'let x = (# ... #) in x.ToString()' leads to better IL, without it, an extra stloc and ldloca.s (get address-of)
              // gets emitted, which are unnecessary. With it, the extra address-of-variable is not created

tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs

@@ -740,6 +740,20 @@ type OperatorsModule2() =
 
     [<Fact>]
     member _.string() =
+
+        let result = Operators.string null
+        Assert.AreEqual("", result)
+
+        let nullStr:string = null
+        let result = Operators.string nullStr
+        Assert.AreEqual("", result)
+
+        let result = Operators.string null
+        Assert.AreEqual("", result)
+
+        let result = Operators.string (null:string)
+        Assert.AreEqual("", result)
+
         // value type
         let result = Operators.string 100
         Assert.AreEqual("100", result)