Fix MPI crash in Observation when allocate_tod=False

[paganol]

This PR addresses a TypeError that occurs when initializing an Observation with allocate_tod=False in an MPI environment.

When allocate_tod is set to False, the Time-Ordered Data (TOD) attributes (e.g., self.tod) are initialized as None. However, the Observation.__init__ method immediately calls setattr_det_global to synchronize detector metadata. Inside setattr_det_global, an assertion attempts to verify the length of the incoming metadata against the length of the TOD array using len().

Because len(None) is an invalid operation, the simulation crashes during the observation creation phase.

Changes:

• litebird_sim/observations.py: Modified setattr_det_global to check if the TOD attribute is None before attempting to validate its length.

• Added a conditional check: the assertion is now only executed if tod_list is present and the corresponding TOD attribute has been allocated.

[github-actions]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
litebird_sim
observations.py					788-799
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