Fix read-chunk in DynamoDBChunkStore

read-a-chunk used to return an empty map, not `nil`, when chunks were not found. As a result, the strongly
consistent read would never be executed, and `(:v {}) => nil` would be returned and NullPointerExceptions of the kind

    Caused by: java.lang.NullPointerException
            at ucar.ma2.Array.factory(Array.java:726)
            at io.mandoline.impl$bytes_to_array.invoke(impl.clj:188)
            at io.mandoline.impl$hash__GT_slab.invoke(impl.clj:193)
            at io.mandoline.impl$get_base_chunk.invoke(impl.clj:212)
            at io.mandoline.impl$update_chunk_BANG_.invoke(impl.clj:224)
            at io.mandoline.impl$write_variable$update_fn__7303.invoke(impl.clj:265)
	    ...

would follow.

This commit fixes the return value of read-a-chunk and adds a simple
unit test.