Properly catch unset preferences

[w-biggs]

With Darktable 5.4.1, when trying to load this script, this error fires:

     5.4318 LUA ERROR: dtutils.lua: prequire: 223: Error loading contrib/nind_denoise_rl 
     5.4319 LUA ERROR: dtutils.lua: prequire: 224: Error returned is bad argument #3 to '?' (number expected, got nil) 
     5.4319 LUA ERROR: script_manager.lua: activate: 541: error loading contrib/nind_denoise_rl 
     5.4319 LUA ERROR: script_manager.lua: activate: 542: error message: bad argument #3 to '?' (number expected, got nil) 

It seems like some API change made it so dt.preferences.read returns empty strings for unset preferences instead of erroring? I'm not 100% sure, but this prevents that from propagating down and causing errors