Avoiding the crash when 'lastModified' is null

The crash was occurring in the method 'dictionaryWithObject' due to an 'NSInvalidArgumentException'.

This is a possible solution to avoid the crash, but other possibilities are:
- Guarantee that the field 'last_modified' is never null  in the table 'appglu_storage_files'.
- When the 'lastModified' is null, update it with the current time.