Fix print.xgb.Booster for R

[r-suzuki]

I always get "Handle is invalid!" message with the R package xgboost 0.71.1. I found that valid_handle is TRUE when x$handle is (a kind of ) NULL, but it seems to be the opposite.

[r-suzuki]

I have also included a fix for is.null.handle to treat NULL handle.

If we load a model object from .RData file (not using xgb.save) the handle object is NULL. While is.null.handle is expected to return TRUE for NULL handle, is.null(handle) is never reached since it stops when checking the class. So I changed the sequence a bit.

So this PR includes:

• Fix print.xgb.Booster to properly check whether handle is NULL or not.
• Avoid stopping the print function when the handle is NULL

[codecov-io]

Codecov Report

Merging #3338 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3338   +/-   ##
=========================================
  Coverage     45.69%   45.69%           
  Complexity      228      228           
=========================================
  Files           166      166           
  Lines         12972    12972           
  Branches        466      466           
=========================================
  Hits           5927     5927           
  Misses         6853     6853           
  Partials        192      192

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[hcho3]

Thanks!