Avoid string allocation when converting zero (#19732)

Author: JamesNK

src/Http/Headers/src/HeaderUtilities.cs

@@ -562,6 +562,11 @@ public unsafe static string FormatNonNegativeInt64(long value)
                 throw new ArgumentOutOfRangeException(nameof(value), value, "The value to be formatted must be non-negative.");
             }
 
+            if (value == 0)
+            {
+                return "0";
+            }
+
             var position = _int64MaxStringLength;
             char* charBuffer = stackalloc char[_int64MaxStringLength];