`get_buffer()` crashes on `None` when using list of PVs

[kabanaty]

When get_buffer() is called with a list of PVs, it uses epics.caget_many() internally. If any PV in the list is unreachable (returns None from caget_many), the method crashes with TypeError: 'NoneType' object is not subscriptable because it attempts to slice None:

buff_dict = {a_pv[:-suffix_length]: buff_dict[a_pv][0:self.n_measurements] for a_pv in buff_dict}

This affects both EventDefinition.get_buffer() (edef.py line 422) and BSABuffer.get_buffer() (sc_buffer.py line 423).

Expected behavior: When a PV in the list can't connect, get_buffer() should return None for that PV in the result dict instead of crashing. This allows callers to handle partial failures gracefully.

Reproduction:

import edef
buffer = edef.EventDefinition("test", user="user", edef_number=3)
# Pass a mix of valid and invalid PVs
buffer.get_buffer(["BPMS:LI24:801:TMIT", "FAKE:INVALID:PV:TMIT"])
# → TypeError: 'NoneType' object is not subscriptable

Proposed fix: Add a None check before slicing in the list comprehension:

buff_dict = {
    a_pv[:-suffix_length]: buff_dict[a_pv][0:self.n_measurements]
    if buff_dict[a_pv] is not None else None
    for a_pv in buff_dict
}