Fix nulable annotations build break (dotnet/coreclr#24768)

Commit migrated from https://github.com/dotnet/coreclr/commit/74d90fdaaa9a0aea2ee5da2cf38dfc07b0bf95fa

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 d563641..74fff87 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -633,14 +633,14 @@ namespace System
                 throw new ArgumentNullException(nameof(values));
             }
 
-            using (IEnumerator<string> en = values.GetEnumerator())
+            using (IEnumerator<string?> en = values.GetEnumerator())
             {
                 if (!en.MoveNext())
                 {
                     return string.Empty;
                 }
 
-                string firstValue = en.Current;
+                string? firstValue = en.Current;
 
                 if (!en.MoveNext())
                 {