If restrictions are an empty string, allow any mime type?

[jpfeuffer]

currently this fails because "" != "mzML" for example.
https://github.com/genericworkflownodes/GenericKnimeNodes/blob/develop/com.genericworkflownodes.knime/src/com/genericworkflownodes/knime/generic_node/GenericKnimeNodeModel.java#L538

Advantage:
This should closer to what one would think it would do.

Disadvantage:
Harder to find errors, where someone just forgot to set a mime type and then the nodes fail at runtime instead of construction time.

Alternative:
Introduce "*" or a placeholder for "any mime type".

Related to #227