[R-package] remove broken default for file in readRDS / saveRDS functions

[jameslamb]

The functions readRDS.lgb.Booster() and saveRDS.lgb.Booster() in the R package take a keyword argument file, which indicates the .rds file to use.

Those functions use default file = "", but this isn't a valid value for that parameter.

readRDS.lgb.Booster():

Error in gzfile(file, "rb") : cannot open the connection
In addition: Warning message:
In gzfile(file, "rb") :
cannot open compressed file '', probable reason 'No such file or directory'

saveRDS.lgb.Booster(list(raw = 1))

Error in saveRDS(object, file = file, ascii = ascii, version = version, :
'file' must be non-empty string

This PR proposes just removing the default value, and forcing users to choose a filepath. I think this is preferable to changing the default value to something like "model.rds", because it makes code using these functions more explicit, and thus less at risk of errors.

While testing this, I created some .rds files and noticed they weren't ignore by git. This PR also changes .gitignore to ignore R's binary formats (.rda, .rds, .RData) so such files aren't accidentally checked into source control.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.