Return input value on empty

src/ImageSharp/Formats/Png/Zlib/Adler32.cs

@@ -59,7 +59,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         {
             if (buffer.IsEmpty)
             {
-                return SeedValue;
+                return adler;
             }
 
 #if SUPPORTS_RUNTIME_INTRINSICS

src/ImageSharp/Formats/Png/Zlib/Crc32.cs

@@ -57,7 +57,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         {
             if (buffer.IsEmpty)
             {
-                return SeedValue;
+                return crc;
             }
 
 #if SUPPORTS_RUNTIME_INTRINSICS

tests/ImageSharp.Tests/Formats/Png/Adler32Tests.cs

@@ -10,6 +10,15 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 {
     public class Adler32Tests
     {
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(2)]
+        public void ReturnsCorrectWhenEmpty(uint input)
+        {
+            Assert.Equal(input, Adler32.Calculate(input, default));
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(8)]

tests/ImageSharp.Tests/Formats/Png/Crc32Tests.cs

@@ -10,6 +10,15 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 {
     public class Crc32Tests
     {
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(2)]
+        public void ReturnsCorrectWhenEmpty(uint input)
+        {
+            Assert.Equal(input, Crc32.Calculate(input, default));
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(8)]