Fix build failure on benchmark server (#1913)

Author: JamesNK

src/Grpc.Core.Api/AuthContext.cs

@@ -101,7 +101,7 @@ public IEnumerable<AuthProperty> Properties
     /// </summary>
     public IEnumerable<AuthProperty> FindPropertiesByName(string propertyName)
     {
-        List<AuthProperty> result;
+        List<AuthProperty>? result;
         if (!properties.TryGetValue(propertyName, out result))
         {
             return Enumerable.Empty<AuthProperty>();

src/Grpc.Core.Api/Internal/CodeAnalysisAttributes.cs

@@ -16,6 +16,8 @@
 
 #endregion
 
+#if !NET5_0_OR_GREATER
+
 // Content of this file is copied with permission from:
 // https://github.com/dotnet/runtime/tree/e2e43f44f1032780fa7c2bb965153c9da615c3d0/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis
 // These types are intented to be added as internalized source to libraries and apps that want to
@@ -148,3 +150,5 @@ internal enum DynamicallyAccessedMemberTypes
     /// </summary>
     All = ~None
 }
+
+#endif

src/Grpc.Core.Api/Metadata.cs

@@ -338,7 +338,7 @@ public byte[] ValueBytes
             {
                 if (valueBytes == null)
                 {
-                    return EncodingASCII.GetBytes(value);
+                    return EncodingASCII.GetBytes(value!);
                 }
 
                 // defensive copy to guarantee immutability
@@ -391,7 +391,7 @@ public override string ToString()
         /// </summary>
         internal byte[] GetSerializedValueUnsafe()
         {
-            return valueBytes ?? EncodingASCII.GetBytes(value);
+            return valueBytes ?? EncodingASCII.GetBytes(value!);
         }
 
         internal bool KeyEqualsIgnoreCase(string key)