Number.new causes IndexError with blank value and whitespace: false

[Blacksmoke16]

$ crystal eval 'pp Float64.new "", whitespace: false'
Unhandled exception: Index out of bounds (IndexError)
  from /usr/lib/crystal/string.cr:996:16 in '[]'
  from /usr/lib/crystal/string.cr:755:40 in 'to_f64?'
  from /usr/lib/crystal/string.cr:710:5 in 'to_f64'
  from /usr/lib/crystal/float.cr:327:5 in 'new:whitespace'
  from eval:1:1 in '__crystal_main'
  from /usr/lib/crystal/crystal/main.cr:118:5 in 'main_user_code'
  from /usr/lib/crystal/crystal/main.cr:104:7 in 'main'
  from /usr/lib/crystal/crystal/main.cr:130:3 in 'main'
  from /usr/lib/libc.so.6 in '??'
  from /usr/lib/libc.so.6 in '__libc_start_main'
  from /home/george/.cache/crystal/crystal-run-eval.tmp in '_start'
  from ???

I'd expect this to bubble up an ArgumentError. At the moment it seems to bubble up an exception from the implementation.

Originally posted by @Blacksmoke16 in #14684 (comment)