Add conditionals for F#9 null handling (#665)

Author: gusty

src/FSharpPlus/Extensions/Obj.fs

@@ -9,7 +9,11 @@ module Obj =
     /// <param name="source">The source object.</param>
     /// <returns>The source if it is not null; otherwise, the specified default value.</returns>
     /// <remarks>The default value is evaluated eagerly.</remarks>
+#if NET9_0_OR_GREATER && !FABLE_COMPILER
+    let defaultValue value (source: 'T | null) : 'T =
+#else
     let defaultValue value (source: 'T) : 'T =
+#endif
         match source with
         | null  -> value
         | value -> value
@@ -19,7 +23,11 @@ module Obj =
     /// <param name="source">The source object.</param>
     /// <returns>The source if it is not null; otherwise, the result of invoking the specified function.</returns>
     /// <remarks>The fNull function is only invoked if the source is null.</remarks>
+#if NET9_0_OR_GREATER && !FABLE_COMPILER
+    let inline defaultWith ([<InlineIfLambda>]fNull: unit -> 'T) (source: 'T | null) : 'T =
+#else
     let inline defaultWith ([<InlineIfLambda>]fNull: unit -> 'T) (source: 'T) : 'T =
+#endif
         match source with
         | null -> fNull ()
         | value -> value
@@ -30,7 +38,11 @@ module Obj =
     /// <param name="source">The source object.</param>
     /// <returns>The result of applying fValue to the source if it is not null; otherwise, the result of applying fNull.</returns>
     /// <remarks>Only one of the functions is invoked based on the nullity of the source.</remarks>
+#if NET9_0_OR_GREATER && !FABLE_COMPILER
+    let inline either ([<InlineIfLambda>]fValue: 'T -> 'U) ([<InlineIfLambda>]fNull: unit -> 'U) (source: 'T | null) : 'U =
+#else
     let inline either ([<InlineIfLambda>]fValue: 'T -> 'U) ([<InlineIfLambda>]fNull: unit -> 'U) (source: 'T) : 'U =
+#endif
         match source with
         | null  -> fNull ()
         | value -> fValue value

src/FSharpPlus/Internals.fs

@@ -49,12 +49,13 @@ module Errors =
     let exnNoSubtraction  = new System.Exception "No subtraction defined for these values in this domain."
     let exnUnreachable    = new System.InvalidOperationException "This execution path is unreachable."
 
-    // Functions to remove when compiling with F#9 or higher
+#if !NET9_0_OR_GREATER || FABLE_COMPILER
     let inline nullArgCheck paramName paramValue =
         if isNull paramValue then nullArg paramName
         else paramValue
 
     module Unchecked = let nonNull = id
+#endif
 
 module Decimal =
     let inline trySqrt x =