Fix null vs empty string mentions in InterpolatedStringHandlerArgumentAttribute docs (dotnet#8674)

Author: stephentoub

xml/System.Runtime.CompilerServices/InterpolatedStringHandlerArgumentAttribute.xml

@@ -49,7 +49,7 @@
 
 ## Remarks
 
-`null` may be used as the name of the receiver in an instance method.
+The empty string may be used as the name of the receiver in an instance method.
 
           ]]></format>
         </remarks>
@@ -86,7 +86,7 @@
 
 ## Remarks
 
-`null` may be used as the name of the receiver in an instance method.
+The empty string may be used as the name of the receiver in an instance method.
 
           ]]></format>
         </remarks>
@@ -116,7 +116,7 @@
 
 ## Remarks
 
-`null` may be used as the name of the receiver in an instance method.
+The empty string may be used as the name of the receiver in an instance method.
 
           ]]></format>
         </remarks>