Bug report #18989
Value for Optional ParameterString can't be defined to None or NULL. It is translated to empty string.
|Affected QGIS version:||2.18.19||Regression?:||Yes|
|Operating System:||All||Easy fix?:||Yes|
|Pull Request or Patch supplied:||Yes||Resolution:||wontfix|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||26820|
In release-2_14, a ParameterString value can be None. So for the R scripts provider, the value is translated to NULL.
In release-2_18, a ParameterString value can't be None, it is translated to empty string. So for the R scripts provider, the value stay an empty string.
So all the R scripts wrote for release-2_14 with optional ParameterString, has to be rewrite to be adapted to this changes.