Added missing length check that caused an ArgumentNullException (#750)

src/ImageSharp/Common/Extensions/EncoderExtensions.cs

@@ -20,6 +20,11 @@ namespace SixLabors.ImageSharp
         /// <returns>The string.</returns>
         public static string GetString(this Encoding encoding, ReadOnlySpan<byte> buffer)
         {
+            if (buffer.Length == 0)
+            {
+                return null;
+            }
+
             fixed (byte* bytes = buffer)
             {
                 return encoding.GetString(bytes, buffer.Length);

tests/ImageSharp.Tests/Common/EncoderExtensionsTests.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Text;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Common
+{
+    public class EncoderExtensionsTests
+    {
+        [Fact]
+        public void GetString_EmptyBuffer_ReturnsNull()
+        {
+            var buffer = new ReadOnlySpan<byte>();
+
+            string result = Encoding.UTF8.GetString(buffer);
+
+            Assert.Null(result);
+        }
+
+        [Fact]
+        public void GetString_Buffer_ReturnsString()
+        {
+            var buffer = new ReadOnlySpan<byte>(new byte[] { 73, 109, 97, 103, 101, 83, 104, 97, 114, 112 });
+
+            string result = Encoding.UTF8.GetString(buffer);
+
+            Assert.Equal("ImageSharp", result);
+        }
+    }
+}