Fix issue where newly created member wasn't logged in after completing an order.

[bummzack]

Fix issue with a PHP error showing up after logout (session was manipulated, but already cleared).

[codecov-io]

Codecov Report

Merging #632 into master will decrease coverage by 0.05%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master     #632      +/-   ##
============================================
- Coverage     46.52%   46.47%   -0.06%     
- Complexity     1582     1584       +2     
============================================
  Files           119      119              
  Lines          4621     4622       +1     
============================================
- Hits           2150     2148       -2     
- Misses         2471     2474       +3

Impacted Files	Coverage Δ	Complexity Δ
src/Extension/MemberExtension.php	96.77% <ø> (-0.11%)	10 <0> (ø)
src/Cart/ShoppingCart.php	79.89% <ø> (ø)	72 <0> (ø)	⬇️
src/Checkout/Component/Membership.php	60.24% <100%> (ø)	32 <0> (ø)	⬇️
src/Checkout/OrderProcessor.php	81.52% <100%> (ø)	52 <0> (ø)	⬇️
src/ShopTools.php	74.19% <66.66%> (-1.67%)	16 <7> (+2)
src/Extension/OrderManipulationExtension.php	46.87% <0%> (-3.13%)	24% <0%> (ø)

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