I would be glad if pgfkeys provided:
- /.blank handler: this is just like /.default but for an empty (or blank) value
- /.default (and /.blank and /.value required for handlers)
I implemented this features rather easily. They DO NOT interact with TikZ.
A minimal example is enclosed (77 lines)
Please ..... ;-)
Minimal pgfkeys example for /.blank handlers and default values for /handlers/