Currently, `sources=[]` by default, which will become "" once the
list is `joined` to form a csv string. The tiingo api appears to
interpret the empty string as "don't match any sources" instead of
"match all sources". This commit fixes the problem by setting
`sources` to `None` if the list is empty.
Adds unit test for empty new sources list.
Updates HISTORY.rst with note about bug fix.