Missing data: skip and leave override unchanged
**Which version of the plugin are you using?**
[x] Data Populator for Sketch
[ ] Data Populator for Adobe XD

**Is your feature request related to a problem? Please describe.**
I'm trying to populate a set of symbols with two different sets of data one after the other. This is necessary because I have one set of data for text and multiple sets for images and I don't want to have the same text data in every image data set because of data maintenance. When I populate with one set of data, the placeholders for which there is no data in the current set, are overridden with substitutes meaning text overrides are deleted and image overrides are replaced by a substitute image.

**Describe the solution you'd like**
I'd like a solution where placeholders with missing data are skipped and overrides remain unchanged. 

**Describe alternatives you've considered**
A workaround for me at the moment is to rename the placeholders temporarily so that they are not recognized when populating. This is not feasible when working with symbols from external libraries – they have to be detached first.

Thank you!