getcwd(): Return empty string if CWD is invalid. #5292

Closes #5291

Restores behaviour identical to Vim. If the user calls the VimScript
function 'getcwd()' and the working directory cannot be found (for
example because the directory has been deleted since the last time it
was used) an empty string needs to be returned instead of throwing an
error.

test/functional/ex_cmds/cd_spec.lua

@@ -269,3 +269,23 @@ for _, cmd in ipairs {'getcwd', 'haslocaldir'} do
   end)
 end
 
+describe("getcwd()", function ()
+  local temp_dir = "Xtest-functional-ex_cmds-cd_spec.temp"
+  before_each(function()
+    clear()
+    lfs.mkdir(temp_dir)
+  end)
+
+  after_each(function()
+    helpers.rmdir(temp_dir)
+  end)
+
+  it("returns empty string if working directory does not exist", function()
+    execute("cd " .. temp_dir)
+    helpers.wait()
+    helpers.rmdir(temp_dir)
+    eq("", helpers.eval("getcwd()"))
+  end)
+end)
+
+