Fixes #key missing for NilClass errors on empty folders

lib/dry/files/memory_file_system.rb

@@ -401,7 +401,7 @@ def entries(path)
         raise IOError, Errno::ENOENT.new(path.to_s) if node.nil?
         raise IOError, Errno::ENOTDIR.new(path.to_s) unless node.directory?
 
-        [".", ".."] + node.children.keys
+        [".", ".."] + Array(node.children&.keys)
       end
 
       private

spec/unit/dry/files/file_system_spec.rb

@@ -609,6 +609,12 @@
       expect(subject.entries(root)).to eq [".", "..", "file-2.txt", "file-1.txt"]
     end
 
+    it "returns an array with only relative paths on an empty directory" do
+      subject.mkdir(root.join("empty"))
+
+      expect(subject.entries(root.join("empty"))).to eq [".", ".."]
+    end
+
     it "raises error if directory doesn't exist" do
       path = root.join("non-existent")
 

spec/unit/dry/files/memory_file_system_spec.rb

@@ -660,5 +660,11 @@ def call(*)
         "file-2.txt"
       ]
     end
+
+    it "returns an array with only relative paths on an empty directory" do
+      subject.mkdir("empty")
+
+      expect(subject.entries(subject.join("empty"))).to eq [".", ".."]
+    end
   end
 end