[cristianchiriac]

Summary

SigmaFilter.from_dict(..., collect_errors=True) is meant to collect parsing
errors into .errors instead of raising, so callers can report every problem
in a filter definition at once. When both the logsource and filter
sections fail to parse, the local variables filter_logsource and
filter_global_filter are never assigned before being passed to cls(...),
so an UnboundLocalError is raised instead — even with collect_errors=True.

from sigma.filters import SigmaFilter
SigmaFilter.from_dict({}, collect_errors=True)
# UnboundLocalError: cannot access local variable 'filter_logsource' ...

Fix

Assign the placeholder values used elsewhere in the codebase for this exact
situation (EmptyLogSource, matching the pattern already used in
SigmaRule.from_dict) in each except branch, plus a new
EmptySigmaGlobalFilter placeholder mirroring EmptySigmaDetections, so the
object can still be constructed while the real errors are recorded in
.errors.

Test plan

• Added test_sigmafilter_collect_errors and
  test_sigmafilter_collect_errors_false_raises in tests/test_filters.py.
• pytest tests/test_filters.py tests/test_correlations.py passes (119 passed, 1 skipped).
• Full suite: 1537 passed, 1 skipped, 2 pre-existing failures unrelated to this change (network/pip access in test_plugins.py).