Set additional Encoding param to be nullable and refactor ValidateArg…

…sAndOpenPath to not require Encoding param anymore like StreamWriter

src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs

@@ -198,7 +198,7 @@ public StreamReader(string path, Encoding? encoding, bool detectEncodingFromByte
         }
 
         public StreamReader(string path, Encoding? encoding, bool detectEncodingFromByteOrderMarks, int bufferSize)
-            : this(ValidateArgsAndOpenPath(path, encoding, bufferSize), encoding, detectEncodingFromByteOrderMarks, bufferSize, leaveOpen: false)
+            : this(ValidateArgsAndOpenPath(path, bufferSize), encoding, detectEncodingFromByteOrderMarks, bufferSize, leaveOpen: false)
         {
         }
 
@@ -224,10 +224,9 @@ private static FileStream ValidateArgsAndOpenPath(string path, FileStreamOptions
             return new FileStream(path, options);
         }
 
-        private static FileStream ValidateArgsAndOpenPath(string path, Encoding encoding, int bufferSize)
+        private static FileStream ValidateArgsAndOpenPath(string path, int bufferSize)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
-            ArgumentNullException.ThrowIfNull(encoding);
             ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
 
             return new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultFileStreamBufferSize);