Treat empty diagrams as errors

[bkryza]

Currently, if the diagram specification results in an empty diagram the result PlantUML or MermaidJS source is still generated, potentially overwriting previous diagram with the same.

By default it should be the case that when diagram is empty, an error is show to the user and the diagram file is not written.

There might be an option to force generation of the diagram source file even when it is empty.