Optimizations to the substr process plugin

Hello

I was recently looking at the substr plugin and noticed it had a couple small issues according to the PHP Inspections (EA Extended) plugin for PHP Storm. The 2 issues are:

1. is_null should not be used any longer. Per the plugin:

All 'is_null(...)' calls can be safely replaced with 'null === ...' constructs (or 'null !== ...' if the original construct was negated).

2. The return statement is redundant:

Variable  $new_value is redundant.   In some cases one-time use variable are simply not needed.

I know these are minor, but every little performance gain helps in the end. Patch to follow in comment.