[MC-99]: Prevent PHP Warning to happen.

Bug fix: [yes]
Feature addition: [no]
Backwards compatibility break: [no]
Phpspec specification passes: [no]
Checkstyle issues: [no]
ChangeLog updated: [no]
Fixes the following issue: PHP Warning come from PriceMappingManager ; check that $absoluteAttributeMapping is not empty before applying max() function to it.