os: add behavior info to `is_dir_empty` documentation (#14110)

vlib/os/os.v

@@ -162,6 +162,7 @@ pub fn rmdir_all(path string) ? {
 }
 
 // is_dir_empty will return a `bool` whether or not `path` is empty.
+// Note that it will return `true` if `path` does not exist.
 [manualfree]
 pub fn is_dir_empty(path string) bool {
 	items := ls(path) or { return true }

vlib/os/os_test.v

@@ -355,6 +355,12 @@ fn test_mv() {
 	assert !os.is_dir(expected)
 }
 
+fn test_is_dir_empty() {
+	// Test that is_dir_empty returns true on
+	// non-existent directories ***as stated in it's doc string***
+	assert os.is_dir_empty('dir that does not exist at all')
+}
+
 fn test_cp_all() {
 	// fileX -> dir/fileX
 	// Note: clean up of the files happens inside the cleanup_leftovers function