Renderer/Html/Array: Fix Undefined offset for callback fixSpaces

[lazyfrosch]

It avoids breaking the callback when no spaces are to be counted.

This is a fixup for 2900dd97eb540488c0bc188a0f5b2b6224a41ddd (#42)

/cc @petrkotek

[TheWitness]

Is this project dead?

[jfcherng]

Is this project dead?

If you don't mind a more modern (PHP >= 7.1 is required and PHP >= 7.4 is preferred) rewritten one: https://github.com/jfcherng/php-diff Benifits: https://github.com/chrisboulton/php-diff/issues/53#issuecomment-455359720

[gullevek]

I second https://github.com/jfcherng/php-diff as a good and working alternative with a dev who actively maintains it and also implements new features!