highlight line should handle case where size == 0

Reproduction instructions:

irb(main):003:0> Parser::CurrentRuby.parse('foo++')
(string):1:6: error: unexpected token $end
(string):1: foo++
(string):1:
Traceback (most recent call last):
...

Note the blank line where there should be either a caret or a tilde indicating the spot of the error.

The code in question is in Parser::Diagnostic::render_line.

I'd submit a pull request, but it isn't clear to me whether the code should be putting out a caret or a tilde in this case. I have verified that column_range.begin_pos is the spot immediately after the end of the line in this case.

Start with the reproduction in the issue, then read lib/parser/diagnostic.rb at Parser::Diagnostic::render_line and inspect how a zero-sized column range is rendered. Decide whether the diagnostic should show a caret or tilde, and verify that the reproduced parse error no longer leaves a blank highlight line.