Constant expression fix

Changed from "A constant expression may be either a value type or a reference type" (with null only under the reference type branch) to "A constant expression shall have the value null, or be of a value type or a reference type." This hoists null to the top level so it's valid regardless of target type — covering nullable value type defaults like void M(int? x = null). Removed the now-redundant null bullet from the reference type list.

Author: BillWagner

standard/expressions.md

@@ -7193,10 +7193,9 @@ constant_expression
     ;
 ```
 
-A constant expression may be either a value type or a reference type. If a constant expression has a value type, that type shall be one of the following: `sbyte`, `byte`, `short`, `ushort`, `int`, `uint`, `nint`, `nuint`, `long`, `ulong`, `char`, `float`, `double`, `decimal`, `bool,` or any enumeration type. If a constant expression has a reference type, the expression shall:
+A constant expression shall have the value `null`, or be of a value type or a reference type. If a constant expression has a value type, that type shall be one of the following: `sbyte`, `byte`, `short`, `ushort`, `int`, `uint`, `nint`, `nuint`, `long`, `ulong`, `char`, `float`, `double`, `decimal`, `bool,` or any enumeration type. If a constant expression has a reference type, the expression shall:
 
-- have a value of type `string`;
-- have a value of `null`; or
+- have a value of type `string`; or
 - be a default value expression ([§12.8.21](expressions.md#12821-default-value-expressions)) of reference type.
 
 A *constant_expression* of type `nint` shall have a value in the range \[-2147483648, 2147483647\]. A *constant_expression* of type `nuint` shall have a value in the range \[0, 4294967295\].