[Export] fetch_nwb assumes fetching table inherits the mixin

[CBroz1]

The use of log_export in fetch_nwb assumes that the query expression has our custom join

spyglass/src/spyglass/utils/dj_helper_fn.py

Lines 238 to 242 in 239f2a5

	nwb_files = (
	query_expression.join(
	tbl.proj(nwb2load_filepath=attr_name), log_export=False
	)
	).fetch(file_name_str)

This should be edited to first run a check that query_expression is an instance of SpyglassMixin