Fix nullable annotation in String.Join

Commit migrated from https://github.com/dotnet/coreclr/commit/06de1100c25eb38e94334054b7176f1baf7e1263

diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
index 75d72dc..d563641 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -626,7 +626,7 @@ namespace System
             }
         }
 
-        public static string Join(string? separator, IEnumerable<string> values)
+        public static string Join(string? separator, IEnumerable<string?> values)
         {
             if (values == null)
             {