Merge branch 'hotfix/1.3.13'
Starting 1.3.14 hotfix branch
Fixed potential warning in rules when user has no groups
Fixed potential exception when passing invalid data to the URL

see https://community.contao.org/de/showthread.php?37668-Produktleser-per-Ajax-updaten&p=243991#post243991
Incorrect rounding for product discount on cart subtotal rules
Products in cart limit was not applied for product rules
Fix foreach error when no options are available (#541)
Fixed sorting buffer issues
Do not create a filter group for keyword search
Added some german labels
Do not query the session storage twice
Must fetch multilingual information of parent product (#465)
Must merge existing product options when available (for variants)
Revert "Do not create a filter group for keyword search"

This reverts commit bf2e99df5bff428fe141078dc30f0b17e5269c38.
Implemented native SQL search for groups, of there are not dynamic attributes
Also use negative sign as keyword split
Fixed error in IsotopeEmail class when passing an array of recipients
Fixed possible empty array bug
Make sure empty keywords and duplicates are removed from search
Update modules.php