Simplify reverse string code, improve algorithm readability

[ammarnajjar]

• swapping the chars with the indecis is obvious, but as it is written to increment (i) right on the next line, I think it is clearer to be precise with words.
• (chars is None) is already included in (not chars) so I would suggest to merge the if statement conditions into one.

[donnemartin]

@ammarnajjar thank you!