Added checking is property not empty string

[idziakjakub]

This change fixes null return when property is empty string.
When property was empty default value has never been returned.

[tmotyl]

This patch also fixed division by zero issue.

[tmotyl]

@dweeves any feedback on this?

[dweeves]

hi, i was hesitating to merge because of possible impact for purposedly empty values in config files for plugins (which inherit from properties).
i don't think there are many (or even if they are any in fact) , but if there are, this will break the plugin testing explicitely for empty value on one of its settings.
So , maybe a better fix would be in magmi_config parsing rather than ignoring empty values in properties .