Fix Convert.FromBase64CharArray with empty array (dotnet/coreclr#10224)

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: stephentoub

src/Common/src/CoreLib/System/Convert.cs

@@ -2713,6 +2713,11 @@ public static Byte[] FromBase64CharArray(Char[] inArray, Int32 offset, Int32 len
 
             Contract.EndContractBlock();
 
+            if (inArray.Length == 0)
+            {
+                return Array.Empty<byte>();
+            }
+
             unsafe
             {
                 fixed (Char* inArrayPtr = &inArray[0])