Errors with empty tokens objects

[koheiw]

> list("a")[0]
list()

> as.tokens(list("a"))[0]
Tokens consisting of 0 document.
 :
Error in h(simpleError(msg, call)) : 
  error in evaluating the argument 'x' in selecting a method for function 'print': subscript out of bounds

> as.tokens(list())
Tokens consisting of 0 document.
Error in qatd_cpp_tokens_recompile(x, type, gap, dup) : 
  Not compatible with STRSXP: [type=NULL].