fix(path): find_upwards() failure case is an infinite loop

lua/plenary/path.lua

@@ -917,14 +917,20 @@ end
 
 function Path:find_upwards(filename)
   local folder = Path:new(self)
-  while self:absolute() ~= path.root do
+  local root = path.root(folder:absolute())
+
+  while true do
     local p = folder:joinpath(filename)
     if p:exists() then
       return p
     end
+    if folder:absolute() == root then
+      break
+    end
     folder = folder:parent()
   end
-  return ""
+
+  return nil
 end
 
 return Path

tests/plenary/path_spec.lua

@@ -720,6 +720,20 @@ SOFTWARE.]]
       assert.are.same(should, data)
     end)
   end)
+
+  describe(":find_upwards", function()
+    it("finds files that exist", function()
+      local p = Path:new(debug.getinfo(1, "S").source:sub(2))
+      local found = p:find_upwards "README.md"
+      assert.are.same(found:absolute(), Path:new("README.md"):absolute())
+    end)
+
+    it("returns nil if no file is found", function()
+      local p = Path:new(debug.getinfo(1, "S").source:sub(2))
+      local found = p:find_upwards "asdf"
+      assert.are.same(found, nil)
+    end)
+  end)
 end)
 
 -- function TestPath:testIsDir()