fix: handling of whitespace-only filenames (#439)

As reported in
TestableIO/System.IO.Abstractions#1070,
whitespace-only files are treated differently depending on the
underlying operating system:
- Windows only supports unicode-whitespace, but throws on filenames with
only blanks
- Linux supports both files with only blanks or with unicode-whitespace

Source/Testably.Abstractions.Testing/FileSystem/FileInfoFactoryMock.cs

@@ -33,7 +33,7 @@ public IFileInfo New(string fileName)
 			throw new ArgumentNullException(nameof(fileName));
 		}
 
-		if (fileName.Trim() == "" && Execute.IsWindows)
+		if (fileName.IsEffectivelyEmpty())
 		{
 			throw ExceptionFactory.PathIsEmpty("path");
 		}

Source/Testably.Abstractions.Testing/Helpers/PathHelper.cs

@@ -32,6 +32,36 @@ internal static bool HasIllegalCharacters(this string path, IFileSystem fileSyst
 			() => false);
 	}
 
+	/// <summary>
+	///     Returns true if the path is effectively empty for the current OS.
+	///     For unix, this is empty or null. For Windows, this is empty, null, or
+	///     just spaces ((char)32).
+	/// </summary>
+	internal static bool IsEffectivelyEmpty(this string path)
+	{
+		if (string.IsNullOrEmpty(path))
+		{
+			return true;
+		}
+
+		return Execute.OnWindows(
+			() => Execute.OnNetFramework(
+				() => string.IsNullOrWhiteSpace(path),
+				() =>
+				{
+					foreach (char c in path)
+					{
+						if (c != ' ')
+						{
+							return false;
+						}
+					}
+
+					return true;
+				}),
+			() => false);
+	}
+
 	internal static bool IsUncPath([NotNullWhen(true)] this string? path)
 	{
 		if (path == null)

Tests/Testably.Abstractions.Tests/FileSystem/FileInfoFactory/Tests.cs

@@ -70,6 +70,42 @@ public void New_ShouldSetLength(string path, byte[] bytes)
 		result.Should().Be(bytes.Length);
 	}
 
+	[SkippableFact]
+	public void New_WithUnicodeWhitespace_ShouldNotThrow()
+	{
+		var exception = Record.Exception(() =>
+		{
+			FileSystem.FileInfo.New("\u00A0"); // Unicode char that's treated as whitespace
+		});
+
+		if (Test.IsNetFramework)
+		{
+			exception.Should().BeOfType<ArgumentException>();
+		}
+		else
+		{
+			exception.Should().BeNull();
+		}
+	}
+
+	[SkippableFact]
+	public void New_WithWhitespace_ShouldThrowOnlyOnWindows()
+	{
+		var exception = Record.Exception(() =>
+		{
+			FileSystem.FileInfo.New("   ");
+		});
+
+		if (Test.RunsOnWindows)
+		{
+			exception.Should().BeOfType<ArgumentException>();
+		}
+		else
+		{
+			exception.Should().BeNull();
+		}
+	}
+
 	[SkippableFact]
 	public void Wrap_Null_ShouldReturnNull()
 	{