Integer field of Code Sources cannot be empty

[Samuel Schluep]

If an integer parameter field in a Code Source that is not required is left empty an error occurs when the Code Source is executed:

[external source element is broken]
error message: invalid literal for int():

It seems, that SilvaExternalSources tries to convert the empty field, either None or an empty string, to an integer and this operation fails.

For example the dLCMS Code Source ms_video has two integer fields, width and height, that are not required. For the moment I have changed the integer fields in ms_video to StringFields. This solves the problem at the moment.

[Andy Altepeter (aaltepet)]

Changed _cast_value to return None if there is no value for the Integer Field.