[nil] is passed to auto_indent_proc when exit with CTRL+d

[tomoya ishida]

Description

require 'reline'
Reline.auto_indent_proc=->(lines,*){raise "lines was [nil]" if lines == [nil]}
Reline.readmultiline('>'){true}

>(press CTRL+d)
sample.rb:2:in `block in <main>': lines was [nil] (RuntimeError)

I expect lines passed to auto_indent_proc to be an array of string.

IRB should specially handle [nil] to not raise error on exit
ruby/irb#515 (comment)

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