[Fix #10737] Fix crash in Style/ConditionalAssignment with EnforcedStyle: assign_inside_condition when op-assigning a variable inside a resbody

[dvandersluis]

Because this cop adds node handlers for all the assignment types, but an op-asgn node contains an lvasgn, the assignment_node method was crashing with:

undefined method `begin_type?' for :bar:Symbol

          assignment, = *assignment if assignment.begin_type? && assignment.children.one?
                                                 ^^^^^^^^^^^^

This change ignores assignments inside op-asgn, or_asgn and and_asgn nodes, as they will have already been covered by the relevant on_* method.

Fixes #10737.

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• Added tests.
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[koic]

Thanks!