Save only string personal doc paths (#1566)

tests/test_personal_docs_lists.py

@@ -0,0 +1,6 @@
+from src import personal_docs
+
+
+def test_string_list_filters_non_strings():
+    assert personal_docs._string_list(["/tmp/a", None, 3, "/tmp/b"]) == ["/tmp/a", "/tmp/b"]
+    assert personal_docs._string_list(None) == []