chore(mbstring): remove PHP 5.6 notices due to deprecated ini setting

In PHP 5.6 the ini setting mbstring.internal_encoding was deprecated. Solution
is to only set it if a value already exists.

Fixes #7244
Merge pull request #7307 from mrclay/7244_mbstring_errors

chore(mbstring): remove PHP 5.6 notices due to deprecated ini setting