Fix bad default in RStudio-prefs.json

[colearendt]

helm/charts/rstudio-workbench/values.yaml

Line 397 in ee5b2a3

rstudio-prefs.json: {}

we should decide whether to null out the default or leave a json blob, etc.

This speaks to a larger question of "mutable types" In helm and how to allow someone to pass either a string or a json blob, etc.

Is this even possible? Related to #35 #379