spec: change unsafe.Slice((*T)(nil), 0) to return []T(nil)

Updates #46742.

Change-Id: I044933a657cd1a5cdb29863e49751df5fe9c258a
Reviewed-on: https://go-review.googlesource.com/c/go/+/331069
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Jun 22, 2021",
+	"Subtitle": "Version of Jun 28, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -6789,11 +6789,17 @@ <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
 </pre>
 
+<p>
+As a special case, if <code>ptr</code> is <code>nil</code> and <code>len</code> is zero,
+<code>Slice</code> returns <code>nil</code>.
+</p>
+
 <p>
 The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
 if it is an untyped constant it is given type <code>int</code>.
-If <code>ptr</code> is <code>nil</code> or <code>len</code> is negative at run time,
+At run time, if <code>len</code> is negative,
+or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
 a <a href="#Run_time_panics">run-time panic</a> occurs.
 </p>
 

src/unsafe/unsafe.go

@@ -221,8 +221,11 @@ func Add(ptr Pointer, len IntegerType) Pointer
 //
 //	(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
 //
+// As a special case, if ptr is nil and len is zero, Slice returns nil.
+//
 // The len argument must be of integer type or an untyped constant.
 // A constant len argument must be non-negative and representable by a value of type int;
 // if it is an untyped constant it is given type int.
-// If ptr is nil or len is negative at run time, a run-time panic occurs.
+// At run time, if len is negative, or if ptr is nil and len is not zero,
+// a run-time panic occurs.
 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType