fix(zip): avoid throwing on empty file name (#828)

Co-authored-by: Dmitrii Makarov <dmakarov@spotware.com>

src/ICSharpCode.SharpZipLib/Core/PathUtils.cs

@@ -16,6 +16,9 @@ public static class PathUtils
 		/// <returns>The path with the root removed if it was present; path otherwise.</returns>
 		public static string DropPathRoot(string path)
 		{
+			// No need to drop anything
+			if (path == string.Empty) return path;
+
 			var invalidChars = Path.GetInvalidPathChars();
 			// If the first character after the root is a ':', .NET < 4.6.2 throws
 			var cleanRootSep = path.Length >= 3 && path[1] == ':' && path[2] == ':';
@@ -26,7 +29,7 @@ public static string DropPathRoot(string path)
 			var cleanPath = new string(path.Take(258)
 				.Select( (c, i) => invalidChars.Contains(c) || (i == 2 && cleanRootSep) ? '_' : c).ToArray());
 
-			var stripLength = Path.GetPathRoot(cleanPath).Length;
+			var stripLength = Path.GetPathRoot(cleanPath)?.Length ?? 0;
 			while (path.Length > stripLength && (path[stripLength] == '/' || path[stripLength] == '\\')) stripLength++;
 			return path.Substring(stripLength);
 		}

test/ICSharpCode.SharpZipLib.Tests/Zip/ZipFileHandling.cs

@@ -1815,5 +1815,27 @@ public void TestDescriptorUpdateOnAdd(UseZip64 useZip64)
 				}
 			}
 		}
+
+		/// <summary>
+		/// Check that Zip files can be created with an empty file name
+		/// </summary>
+		[Test]
+		[Category("Zip")]
+		public void HandlesEmptyFileName()
+		{
+			using var ms = new MemoryStream();
+			using (var zos = new ZipOutputStream(ms){IsStreamOwner = false})
+			{
+				zos.PutNextEntry(new ZipEntry(String.Empty));
+				Utils.WriteDummyData(zos, 64);
+			}
+			ms.Seek(0, SeekOrigin.Begin);
+			using (var zis = new ZipInputStream(ms){IsStreamOwner = false})
+			{
+				var entry = zis.GetNextEntry();
+				Assert.That(entry.Name, Is.Empty);
+				Assert.That(zis.ReadBytes(64).Length, Is.EqualTo(64));
+			}
+		}
 	}
 }