Handle uninitialized cache object

[shashank88]

If _cache is not initialized - I have seen this in some contexts with forking involved - just return the uncached list_libraries.

[luocy16]

What is this bug?

File "create_dataset_new.py", line 100, in add_one_stock_daily_data_wrapper
error_index_list = add_one_stock_daily_data(filepath, type, exchange_place, arc, date)
File "create_dataset_new.py", line 86, in add_one_stock_daily_data
lib.update(symbol, df, chunk_size="D")
File "/home/shared_user/anaconda3/envs/luocy2/lib/python3.8/site-packages/arctic/chunkstore/chunkstore.py", line 567, in update
self.__update(sym, item, metadata=metadata, combine_method=lambda old, new: new, chunk_range=chunk_range, audit=audit)
File "/home/shared_user/anaconda3/envs/luocy2/lib/python3.8/site-packages/arctic/chunkstore/chunkstore.py", line 433, in __update
df = self.read(symbol, chunk_range=chunker.to_range(start, end), filter_data=False)
File "/home/shared_user/anaconda3/envs/luocy2/lib/python3.8/site-packages/arctic/chunkstore/chunkstore.py", line 287, in read
return deser(chunks[symbol[0]], **kwargs) if skip_filter else chunker.filter(deser(chunks[symbol[0]], **kwargs), chunk_range)
File "/home/shared_user/anaconda3/envs/luocy2/lib/python3.8/site-packages/arctic/serialization/numpy_arrays.py", line 220, in deserialize
index = INDEX in meta
TypeError: argument of type 'NoneType' is not iterable