bug: JSON parameters aren't copied over for Image Collection Custom Dialog

[zacdezgeo]

I've been running into some hard system crashes when copying the json parameters or qgis_process command of the image collection dialog.

The problem may be related to the types of parameters returned, the interface may expect a serialized dictionary and not complex types.

[zacdezgeo]

We added a TODO in #252; however, even when returning an empty string in createProcessingParameters for the BaseAlgorithmDialog(gui.QgsProcessingAlgorithmDialogBase), QGIS immediately crashes with a nested error.

@gena @aSullivan-geo; this would only impact our algorithms that implement a custom dialog widget (the image collection for now). The number of users who would copy JSON settings is probably limited. I don't have an estimate of the time to resolve. I'd suggest we deprioritize it for the time being.

[zacdezgeo]

Just updating, this no longer crashes on the dev branch. However, the output dictionary is empty. I've spent a little time digging into this, but didn't find an immediate fix.