fix(io): crash if getMultiFrameGroupSequence is not found
Description
When trying to access to a private sequence of a multi frame, the result of getMultiFrameGroupSequence can be nullopt.
This value isn't tested and can lead to unexpected crashes.
Steps to reproduce
Try to access to getMultiFrameGroupSequence an a series that doesn't have one.
Proposal
Test the optional value, if nullopt return empty sequence.
Functional specifications
This function should not crash.
Technical specifications
See proposal
Test plan
Add this case in the unit test SeriesTest