[4.2] Fixes mutators implementation when $mutatorCache for given class
is not available.

Solves #6769

Signed-off-by: crynobone <crynobone@gmail.com>
Additional fixes based on comment.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge pull request #6773 from crynobone/patch/mutator

[4.2] Fixes mutators implementation when $mutatorCache for given class is not available.