Fix MemoryFileSystem to treat an empty path as non-existent (#213)

* Fix MemoryFileSystem to treat an empty path as non-existent

`_MemoryFileSystem.findNode` returns `reference?.directory` if given
the root path or an empty path.  However, an empty path should
be treated as non-existent, not as a directory.  Instead explicitly
return `null` as a special case when given an empty path.

Partially addresses https://github.com/google/file.dart/issues/198.

* Make _MemoryFileSystem._current nullable

packages/file/CHANGELOG.md

diff --git a/packages/file/CHANGELOG.md b/packages/file/CHANGELOG.md
index dd09a11..6e46da0 100644
--- a/packages/file/CHANGELOG.md
+++ b/packages/file/CHANGELOG.md
@@ -1,6 +1,7 @@
 #### 7.0.0-wip
 
 * Dart 3 fixes for class modifiers.
+* `MemoryFileSystem` now treats empty paths as non-existent.
 * Fix `FileSystem.isLink`/`FileSystem.isLinkSync` to not follow symbolic links.
 
 #### 6.1.4

packages/file/lib/src/backends/memory/memory_file_system.dart

diff --git a/packages/file/lib/src/backends/memory/memory_file_system.dart b/packages/file/lib/src/backends/memory/memory_file_system.dart
index 8d63ea5..f3cdaee 100644
--- a/packages/file/lib/src/backends/memory/memory_file_system.dart
+++ b/packages/file/lib/src/backends/memory/memory_file_system.dart
@@ -200,7 +200,7 @@
   /// Gets the node backing for the current working directory. Note that this
   /// can return null if the directory has been deleted or moved from under our
   /// feet.
-  DirectoryNode get _current => findNode(cwd) as DirectoryNode;
+  DirectoryNode? get _current => findNode(cwd) as DirectoryNode?;
 
   @override
   Node? findNode(
@@ -211,7 +211,9 @@
     List<String>? pathWithSymlinks,
     bool followTailLink = false,
   }) {
-    if (_context.isAbsolute(path)) {
+    if (path.isEmpty) {
+      return null;
+    } else if (_context.isAbsolute(path)) {
       reference = _root;
       path = path.substring(style.drive.length);
     } else {

packages/file/test/common_tests.dart

diff --git a/packages/file/test/common_tests.dart b/packages/file/test/common_tests.dart
index 679a90a..22fb36f 100644
--- a/packages/file/test/common_tests.dart
+++ b/packages/file/test/common_tests.dart
@@ -370,6 +370,11 @@
       });
 
       group('stat', () {
+        test('isNotFoundForEmptyPath', () {
+          FileStat stat = fs.statSync('');
+          expect(stat.type, FileSystemEntityType.notFound);
+        });
+
         test('isNotFoundForPathToNonExistentEntityAtTail', () {
           FileStat stat = fs.statSync(ns('/foo'));
           expect(stat.type, FileSystemEntityType.notFound);