MINOR: flt-decomp: Don't instantiate a filter without applicable direction

MINOR: flt-decomp: Don't instantiate a filter without applicable direction

decomp_strm_init() asserted that the filter configuration was never
NULL. This holds for the legacy filter configurations, which always carry
the decompression direction of their proxy. But an implicit instance
inherited by a proxy without the matching capability will keep its metadata
while having no applicable direction, leaving its configuration NULL when
the instance is finalized.

Drop the assertion and handle a NULL configuration like a direction disabled
with the "none" mode or without any algorithm: the filter is not attached to
the stream and the instance remains as metadata. The decompression behavior
is unchanged, such a filter was already ignored on every stream.